Help! Betta Keeps Shredding Fins

Hi! I have a Betta fish named Squeak and he keeps on shredding up his fins. He's in a three gallon tank with gravel, driftwood, creeping Charlie, Java fern, Java moss, a Marino moss ball, an almond leaf, a banana plant, with some snails and a Cory for company. I change the water every Sunday with water conditioner. He just started doing this a few weeks ago, and I Dont know why.

At first I thought he was getting his fins sucked into the filter so I switched it out for a HOB filter with a sponge on the intake. I don't think the flow is too strong, and he has no trouble swimming.
The new filter didn't help much, so then I thought it might be fin biting. I read people have stopped that by re-aranging decor, getting a tank-mate, and putting almond leaves, aquarium salt, and stress coat in the water. I did all of those things and he STILL shreds his fins somehow over and over again. As soon as they start healing they are shredded again overnight. He didn't use to do this.

I test the water with test strips

Ammonia: .5
Nitrite: 0
Nitrate: 15
Ph: 6.8
Hardness: 150
Chlorine: 0
Alkalinity: 0

The other day he somehow got a hole near the base of his tail fin and the next morning a whole Ray was missing!
I don't think it could be fin rot because it is so sudden and happens overnight or just in a few hours during the day. Apart from his fins he's a very happy healthy and very active fish.

Here are some pics of him and his tank:

















Any advise is appreciated,

Lucy & Squeak

 

Welcome to Fishlore! It appears he has fin rot. I would do a large water change to get the ammonia level down. Corydoras are schooling fish that should be in groups of a bare minimum of 5 and a 20 gallon (preferably long) tank.

 

It could be the cause of stress, pent up aggression, or ragged fins. There are more causes, but I will be naming these three. If you would like more causes, ask me. Ragged fins are caused by bad quality water in the fish tank. If you think this is the cause, try cleaning your tank water.
He jumped out, did he? Try removing everything and looking for him. Or, he may have jumped out. Hope this helps ^^

 

Welcome to Fishlore! It appears he has fin rot. I would do a large water change to get the ammonia level down. Corydoras are schooling fish that should be in groups of a bare minimum of 5 and a 20 gallon (preferably long) tank.

Thanks for the reply
I already do a 75% water change with a gravel vacuum once a week, should I do more? Sometimes his fins get ripped up within an hour or two, could fin rot really happen that fast? I don't see any discoloration on the tips of his fins or anything

 

Thanks for the reply
I already do a 75% water change with a gravel vacuum once a week, should I do more? Sometimes his fins get ripped up within an hour or two, could fin rot really happen that fast? I don't see any discoloration on the tips of his fins or anything

I don't really see anything his fins could get caught on. I would do two 50-60 percent water changes weekly in a tank that size.

 

Does your tank have any places for your betta to hide in? The tank light looks pretty bright, and that can potentially stress out a betta if they feel like they're constantly exposed and don't have a safe spot to stay at to feel secure. My delta tail used to shred his fins at night since I'd have a light on while working at my desk, but he stopped that when I put a piece of cardboard over the tank so he doesn't get so much light in the evenings. Fish needs his beauty sleep!

I agree with the water changes, since if I recall correctly, you want ammonia as close to 0 as possible in a betta tank. Is there a chance you could get a test kit or something similar that's more accurate than the strips?

 

Beautiful betta! It could very well be the ammonia stressing him out. Switching filters may have disrupted your tank's nitrogen cycle, so I'd be doing large water changes to get the ammonia down, and dosing with Seachem Prime water conditioner to protect him from the ammonia. It's not crazy high, but enough to cause irritation. Bettas with huge fins will sometimes bite them for reasons we can only speculate about...

 

This looks like fin rot. To cure this, many fish stores sell a solution that usually works. Your tank is beautiful by the way!

 

Does your tank have any places for your betta to hide in? The tank light looks pretty bright, and that can potentially stress out a betta if they feel like they're constantly exposed and don't have a safe spot to stay at to feel secure. My delta tail used to shred his fins at night since I'd have a light on while working at my desk, but he stopped that when I put a piece of cardboard over the tank so he doesn't get so much light in the evenings. Fish needs his beauty sleep!

I agree with the water changes, since if I recall correctly, you want ammonia as close to 0 as possible in a betta tank. Is there a chance you could get a test kit or something similar that's more accurate than the strips?

The driftwood makes a little tunnel/cave underneath, but he only seems to use it at night to sleep and isn't that interested in it during the day. The light is pretty bright though because of the plants, so maybe that's making him stressed. Should I turn the light off earlier in the evening? Maybe he can see his reflection in the glass or something.
I could get a Betta log if that would help him, but there's not much room in the tank for extra decor.
I have a liquid test kit but the last time I tried to use it I spilled a little bit and it burned my thumb! That's why I got the strips. I did a water change yesterday and the ammonia is at 0.ppm now, according to the strips.

What puzzles me is why he suddenly started doing this, because he had been in this tank for several months before any fin problems arose.

Thanks for the reply
Lucy

Beautiful betta! It could very well be the ammonia stressing him out. Switching filters may have disrupted your tank's nitrogen cycle, so I'd be doing large water changes to get the ammonia down, and dosing with Seachem Prime water conditioner to protect him from the ammonia. It's not crazy high, but enough to cause irritation. Bettas with huge fins will sometimes bite them for reasons we can only speculate about...

He is a pretty little fish will API stress coat water conditioner protect him from the ammonia? I just got a bottle last week.
It's just odd to me that he's been in the same tank on the same schedule for several months and only just started having tail problems.
Could he be biting his tail? His dorsal fin also has rips though and I doubt he could reach

 

100% not a tail biter, unless he is secretly a fine barber and gives himself a very even fin cut.

Fin rot, probably exclerated by stress of having a "fiend" and strong filtration. Is that a HOB I see? Flow contributes to ruining betta fins.
For quick fix - separate into a bare shoe tote with maybe one hammock, and dose with either low dose of betta revive or very mild salt.

For best and fastest results - and I recommend this with caution and very strict daily wc - remove all filtation for a few weeks after.

 

100% not a tail biter, unless he is secretly a fine barber and gives himself a very even fin cut.

Fin rot, probably exclerated by stress of having a "fiend" and strong filtration. Is that a HOB I see? Flow contributes to ruining betta fins.
For quick fix - separate into a bare shoe tote with maybe one hammock, and dose with either low dose of betta revive or very mild salt.

For best and fastest results - and I recommend this with caution and very strict daily wc - remove all filtation for a few weeks after.

I don't think having a friend in the tank would be very stressful, and he really seems to like the other fish. He sleeps with it in the corner every day and follows it around everywhere Anyway his fin problems started before the new tankmate arrived.
Also the filtration really isn't that strong. I have the filter on high just to make a slight disturbance on the water surface and oxygenate the water.
Isn't it a little extreme to quarantine and medicate him so soon? I'm not very knowledgable about what fin rot looks like but I thought the tips of the fins changed colors. I would like to take a more natural approach if possible.
I can probably make time to do more water changes and I was putting aquarium salt in the water, but I stopped that after I got the tankmate because I read that it wasn't good for it.
What's a bare shoe tote?
Also if I remove the filtration won't the beneficial bacteria die?

Thanks!
Lucy

 

Yes, yes and yes to all of that - though just keep the filter running in the main tank and out the betta in a plastic container (shoe tote). If you want a quick fix-it, that's what will work.

You know about water changes so what is the point of advising that? 20% water change will slowly help - provided your water is perfect. It's not bad fin rot, though it is deep, and at the very least that will stop it.

Male bettas are not friends to other fish. Never. Everything is fine until one day someone dies. Corydoras are schooling fish, so maybe he is stalking the betta to feel safer - which is the opposite of safety. Also, Cory's are bottom swimmers and bettas are top. To me, hearing that they sleep together and follow each other is alarming. A peaceful betta tank is a tank where the betta 100% ignores his tank mates.

 

The immediate problem I spotted was that the ammonia is at 0.5, and although bettas are less likely to get sick when there is a little bit of ammonia (not too much) doesn't mean they won't. Especially when it comes to their fins. Although they can breathe air from outside their exteriors are still exposed to the ammonia.

Other than that sometimes its the betta's genetics. I got a betta that I thought for sure had some form of fin rot. I tried to treat him and do whatever I could because his fins looked so miserable. Nothing I tried ever worked. But he's lived like that for 3 years and is happy and otherwise healthy. He's a very active swimmer and I think that might also contribute to it. Most bettas I see are kind of lazy or just rather slow when they swim. Mine likes to sit in the current and swim towards it. He also likes to race around the tank.

Yes, yes and yes to all of that - though just keep the filter running in the main tank and out the betta in a plastic container (shoe tote). If you want a quick fix-it, that's what will work.

You know about water changes so what is the point of advising that? 20% water change will slowly help - provided your water is perfect. It's not bad fin rot, though it is deep, and at the very least that will stop it.

Male bettas are not friends to other fish. Never. Everything is fine until one day someone dies. Corydoras are schooling fish, so maybe he is stalking the betta to feel safer - which is the opposite of safety. Also, Cory's are bottom swimmers and bettas are top. To me, hearing that they sleep together and follow each other is alarming. A peaceful betta tank is a tank where the betta 100% ignores his tank mates.

I have cory cats that love my betta. My betta is a very active swimmer and loves to explore the 30 g tank. He'll go around where the cory cats are and the corys will shoal with him like he's another cory. He's never bothered another fish. As soon as I add a new fish he goes up to them while they're still in the bag, flares, stops flaring and swims around them, and then never tries to intimidate them again. He pretty much ignores them afterwards. I've never had him have a problem with attacking another fish. I do have another betta who is isolated in a 3.5g tank. He can't be around any other fish or invert. He tries to attack everything. It really does depend on the personality of the betta, and most of the time you see warning signs before "someone dies".

 

Yes, yes and yes to all of that - though just keep the filter running in the main tank and out the betta in a plastic container (shoe tote). If you want a quick fix-it, that's what will work.

You know about water changes so what is the point of advising that? 20% water change will slowly help - provided your water is perfect. It's not bad fin rot, though it is deep, and at the very least that will stop it.

Male bettas are not friends to other fish. Never. Everything is fine until one day someone dies. Corydoras are schooling fish, so maybe he is stalking the betta to feel safer - which is the opposite of safety. Also, Cory's are bottom swimmers and bettas are top. To me, hearing that they sleep together and follow each other is alarming. A peaceful betta tank is a tank where the betta 100% ignores his tank mates.

Can fin rot really shred a fin in an hour? I though it slowly ate away at the fin, so I can't see how it could be fin rot if I see his fins are normal when I feed him in the morning and then an hour later I check in on him and the fin is shredded. It also seems kind of sporadic, a few days where nothing happens to the fins, and then one day it is ripped. The ammonia is at 0 now after I did a water change.
I don't want to argue, but I completely disagree about bettas never ever being friends with any fish. It depends on the bettas personality, and I think he likes this fish. He's very peaceful.
How could a whole fin Ray rip off overnight? Is that really what fin rot looks like?
Thanks
Lucy

 

Not arguing! You asked for advise and here it is. Feel free to disagree, it's your fish and I can't fix it for you.

Here's a rundown on fin rot in bettas:

Those tattered/frayed ends on the edge of the fins are fin rot.

Fin rot damages the edges of the fin, which, once weakened, can and do split between the rays and will make the edges prone to more fin rot in conditions like: a). Strong filtration
b). High ammonia water
c). Rough decor
d). Stressed fish that paces the tank
e). Extreme activity

Pick your poison. I don't think it's some of those things like rough decor for example. Even if it was an accident like bumping into things, something caused that, and something caused the fin rot.

 

Could I mention something?
The temperature is the tank appearing to be 69F. That is freezing to bettas, which should be kept at 78-82 constant.

 

Not arguing! You asked for advise and here it is. Feel free to disagree, it's your fish and I can't fix it for you.

Here's a rundown on fin rot in bettas:

Those tattered/frayed ends on the edge of the fins are fin rot.

Fin rot damages the edges of the fin, which, once weakened, can and do split between the rays and will make the edges prone to more fin rot in conditions like: a). Strong filtration
b). High ammonia water
c). Rough decor
d). Stressed fish that paces the tank
e). Extreme activity

Pick your poison. I don't think it's some of those things like rough decor for example. Even if it was an accident like bumping into things, something caused that, and something caused the fin rot.

Thanks for the advise
He seems to like swimming into the current, but when he's not right under the filter it doesn't affect him at all. Would swimming into the filter current be bad for his fins? He is very active and paces the front edge of the tank all of the time. You said that was part of fin rot or something, is it bad for him to do that?
Lucy

 

^oh yes, deifintiely too cold. O_O Where do you see a thermometer?

 

Could I mention something?
The temperature is the tank appearing to be 69F. That is freezing to bettas, which should be kept at 78-82 constant.

He has not had a heater his whole life, and he's been fine. If it was the temperature wouldn't the problem have started a long time ago?

 

He has not had a heater his whole life, and he's been fine. If it was the temperature wouldn't the problem have started a long time ago?

Regardless, he needs a heat source, with water at 78-82.
It can eventually become an issue with health. Keeping him at cold temps will shorten his life.

 

I just don't get why this suddenly started happening when he's been in this tank on this schedule for ages

 

Oh dear.

No, you need a heater. That's no good. He definitely isn't fine - he literally can't be, not at that temp. Even if he surviving, it is just barely, and living like that will cut his life span significantly.

There's your problem, then.

 

Oh dear.

No, you need a heater. That's no good. He definitely isn't fine - he literally can't be, not at that temp. Even if he surviving, it is just barely, and living like that will cut his life span significantly.

There's your problem, then.

I was going to get a heater for the winter, but does he really need one for the summer when the water is at 72 most of the time?
It makes me really nervous at the idea of a heater under water, in the reviews of lots of heaters people say there was an electrical short and it electrocuted the fish. I really don't want that to happen
I'm pretty sure it's not 80 in Thailand 24/7

 

He needs 78 minimum at all times. He is freezing.

Think of it this way: imagine you've been eating 50 bars of chocolate a day all your life and "had no problems." Then, "suddenly" you got diabetus. That's not sudden at all.

 

Do they make heaters that don't go in the water?
How do they ever survive the winter in the wild?

 

Yes, yes and yes to all of that - though just keep the filter running in the main tank and out the betta in a plastic container (shoe tote). If you want a quick fix-it, that's what will work.

You know about water changes so what is the point of advising that? 20% water change will slowly help - provided your water is perfect. It's not bad fin rot, though it is deep, and at the very least that will stop it.

Male bettas are not friends to other fish. Never. Everything is fine until one day someone dies. Corydoras are schooling fish, so maybe he is stalking the betta to feel safer - which is the opposite of safety. Also, Cory's are bottom swimmers and bettas are top. To me, hearing that they sleep together and follow each other is alarming. A peaceful betta tank is a tank where the betta 100% ignores his tank mates.

I am currently housing cories in a 25 gallon tank (upgraded my 10 to a 25), and moved my cories and bettas over... 5 Albino's & 1 Betta... My betta never bothers them ever -- but, likes to sleep with them, every night in the same spot, it's very odd.

He has not had a heater his whole life, and he's been fine. If it was the temperature wouldn't the problem have started a long time ago?

Betta's are tropical fish, therefore, need a heater -- without a heater, sooner or later your betta will run into problems like this as they get worn down, and weak, therefore making them an easy target to disease.

Despite being hardy fish, they are very fragile to temperature & water quality -- because you are housing a betta in a 3 Gallon with a cory (depending on the cory, IF Albino needs way bigger room then that.) You are needing to do more frequent water changes... A 3 Gallon can barely keep up with a betta.. The cory as an addition will effect water quality regardless..

Seems like your water quality is the issue, you have mentioned to someone that you swapped the filters... Because of this like someone stated, you may of destroyed your cycle and are in the process of cycling again which will be hard on your betta.

I recommend re-homing the cory, if you chose not to, then, increase your water changes (smaller ones, more PER week).
For now, I also recommend doing 20% - 30% (around that mark area) water changes about 2-3 x per week AS your water quality isn't the best and IF you are in the process of cycling again.

Test strips are not accurate, start using your liquid testing kid for more accurate results to see what it is going on. I also recommend getting a heater, by getting a heater, you will get rid of fin rot --the fins will slowly repair with GOOD water quality & a stable temperatures of 78 - 79.

Hope that helps. Since it's a 3 Gallon, I'd recommend the Hydor heater. Most reliable & you can adjust.

(Also, I forgot to mention, the only reason he was doing OK without a heater IS because they are decently hardy fish... Despite this, it takes a couple months or so, till they are worn out and the lack of heater effects catch up to them.) They are tropical fish.

 

Do they make heaters that don't go in the water?
How do they ever survive the winter in the wild?

Just buy a heater. It's $10 at Walmart. If you don't want it to go in the water because you are worried about the swimming room or that your tank is your small - it is lol, you're right to worry.

They are tropical fish. 76 degrees is tropical winter, and you have it at way below that. Can't be good for him.

 

By not doing something actually beneficial for your pet, you're actually hurting him, whether he's alive or not.

 

Your tank isn't big enough for 2 fish. You need to up your water changes to 50% twice a week minimum.

Your fish does have fin rot. I would treat with nitrofurazone. Either Jungle Fungus Clear tabs from Walmart or Furan-2.

I would rehome the corycat. 3 gallon is much much much too small for him.

You need a heater. 69 is too cold. You are currently having problems. Probably because of the temperature. It has lowered his immune system, causing an illness. So yes a heater is necessary.

 

Betta's are tropical fish, therefore, need a heater -- without a heater, sooner or later your betta will run into problems like this as they get worn down, and weak, therefore making them an easy target to disease.

Despite being hardy fish, they are very fragile to temperature & water quality -- because you are housing a betta in a 3 Gallon with a cory (depending on the cory, IF Albino needs way bigger room then that.) You are needing to do more frequent water changes... A 3 Gallon can barely keep up with a betta.. The cory as an addition will effect water quality regardless..

Seems like your water quality is the issue, you have mentioned to someone that you swapped the filters... Because of this like someone stated, you may of destroyed your cycle and are in the process of cycling again which will be hard on your betta.

I recommend re-homing the cory, if you chose not to, then, increase your water changes (smaller ones, more PER week).
For now, I also recommend doing 20% - 30% (around that mark area) water changes about 2-3 x per week AS your water quality isn't the best and IF you are in the process of cycling again.

Test strips are not accurate, start using your liquid testing kid for more accurate results to see what it is going on. I also recommend getting a heater, by getting a heater, you will get rid of fin rot --the fins will slowly repair with GOOD water quality & a stable temperatures of 78 - 79.

Hope that helps. Since it's a 3 Gallon, I'd recommend the Hydor heater. Most reliable & you can adjust.

(Also, I forgot to mention, the only reason he was doing OK without a heater IS because they are decently hardy fish... Despite this, it takes a couple months or so, till they are worn out and the lack of heater effects catch up to them.) They are tropical fish.

thanks for the heater recommendation, I'll look that up

Just buy a heater. It's $10 at Walmart. If you don't want it to go in the water because you are worried about the swimming room or that your tank is your small - it is lol, you're right to worry.

They are tropical fish. 76 degrees is tropical winter, and you have it at way below that. Can't be good for him.

Ok, I'll get a heater, but do you know of any that go out of the water? I read some reviews and some people said the heater malfunctioned and electrocuted their fish. I really don't want that to happen.
Do I need to medicate him or will it heal with just warm clean water? I would prefer to not do any medicines if that's possible, but I do have some erythromycin if that's any good.
Would 10 gallons be enough for a cory?

Thanks

 

Ok, I'll get a heater, but do you know of any that go out of the water? I read some reviews and some people said the heater malfunctioned and electrocuted their fish. I really don't want that to happen.
Do I need to medicate him or will it heal with just warm clean water? I would prefer to not do any medicines if that's possible, but I do have some erythromycin if that's any good.
Would 10 gallons be enough for a cory?

Thanks

Your heater will be just fine. It probably won't malfunction.
Pretty sure the fins will heal with clean water, and warm temps.
20g long for cory.

 

Your heater will be just fine. It probably won't malfunction.
Pretty sure the fins will heal with clean water, and warm temps.
20g long for cory.

Corys are so small, why do they need 20 gallons? they are smaller than betta fish!

 

Ok, I'll get a heater, but do you know of any that go out of the water? I read some reviews and some people said the heater malfunctioned and electrocuted their fish. I really don't want that to happen.
Do I need to medicate him or will it heal with just warm clean water? I would prefer to not do any medicines if that's possible, but I do have some erythromycin if that's any good.

Thanks

Get a good quality brand that is adjustable. Heaters malfunctioning is very rare. Normally they just stop working. Aqueon or eheim are the top brands for heaters. You basically get what you pay for. Get a top brand, and you will be happy. They have to go inside the water. They won't be able to heat the water any other way. Millions of fishkeepers with submersible heaters. And only a small handful of problems. The heaters are perfectly safe.

You need to do the meds. It is an active infection now. It needs to be treated. You will need to do 2 rounds of the medication.

Do a 75% water change. Remove the carbon from your filter. Put med in tank. Wait 4 days. Do another 75% water change. Put the med in the tank. Wait 4 days. Do another 75% water change and put carbon back in your filter. If you have a filter cartridge, you may need to make a small slit in the cartridge to dump the carbon out. Just put the emptied cartridge back into the filter. Do not throw it away.

Do not add anymore salt to your tank. Corydoras are very sensitive to salt. It can burn him causing severe injuries.

 

Corys are so small, why do they need 20 gallons? they are smaller than betta fish!

Don't assume the tank size by the size of your fish.
Cory are extremely active swimmers, and need lots of room to swim around.

Edit: Proving this, one of my cory have reached the 3" mark. She grew half an inch when introduced into the cycled community tank. That tank is 31 gallons.

 

Get a good quality brand that is adjustable. Heaters malfunctioning is very rare. Normally they just stop working. Aqueon or eheim are the top brands for heaters. You basically get what you pay for. Get a top brand, and you will be happy. They have to go inside the water. They won't be able to heat the water any other way. Millions of fishkeepers with submersible heaters. And only a small handful of problems. The heaters are perfectly safe.

You need to do the meds. It is an active infection now. It needs to be treated. You will need to do 2 rounds of the medication.

Do a 75% water change. Remove the carbon from your filter. Put med in tank. Wait 4 days. Do another 75% water change. Put the med in the tank. Wait 4 days. Do another 75% water change and put carbon back in your filter. If you have a filter cartridge, you may need to make a small slit in the cartridge to dump the carbon out. Just put the emptied cartridge back into the filter. Do not throw it away.

Do not add anymore salt to your tank. Corydoras are very sensitive to salt. It can burn him causing severe injuries.

I stopped the salt already, I read it was bad for corys. what medication do I need? I heard on other posts about fin rot that it will heal with just clean warm water.

Don't assume the tank size by the size of your fish.
Cory are extremely active swimmers, and need lots of room to swim around.

Edit: Proving this, one of my cory have reached the 3" mark. She grew half an inch when introduced into the cycled community tank. That tank is 31 gallons.

I would say my betta is at least as active as the cory though

 

No. You are past that point. You now have pinhole fin rot. It has to be treated. You need to use either Jungle Fungus Clear tabs or Furan-2. Both meds contain Nitrofurazone. So either one will work. The jungle Fungus Clear tabs you can find at Walmart.

If you are keeping the corycat at the very least upgrade the tank to a 10 gallon. That way he can have room to swim around. What kind is he? Some of them aren't temperature compatible with bettas. The lowest temp for bettas is 78 whereas some of the highest temps for cories is around 75. Depending on the species.

 

No. You are past that point. You now have pinhole fin rot. It has to be treated. You need to use either Jungle Fungus Clear tabs or Furan-2. Both meds contain Nitrofurazone. So either one will work. The jungle Fungus Clear tabs you can find at Walmart.

If you are keeping the corycat at the very least upgrade the tank to a 10 gallon. That way he can have room to swim around. What kind is he? Some of them aren't temperature compatible with bettas. The lowest temp for bettas is 78 whereas some of the highest temps for cories is around 75. Depending on the species.

This might be a dumb question, but how do you know it is fin rot? If it was, wouldn't his fins be rotting away? I see no signs of fin rot like discoloration or fin loss. He hasnt lost any fin as far as I can tell. Would fin rot really explain how a whole ray came out of his tail fin? It just doesn't seem like fin rot to me. I thought pinhole fin rot was where they get tiny holes in the fins and I don't see any. how could fin rot make fins rip but not do anything else? I'm confused

 

See how the tips of his fins are all matted together. That is called ribboning. It is a sign of fin rot. The fins should not stick together like that on the ends. It only does that when it is infected. Plus when you look at the first photo, you see that hole in the tail fin. That is pin hole fin rot. That isn't a tear. This is fin rot. Treat it or don't. Up to you at this point.

 

Please just listen and learn. You are getting good advice from experts in the above posts. It's great that you are asking questions and learning though! The heater is a requirement for long term health. It's a common mistake because people see them in the store in little unheated cups and think that's ok. It's really not ok, and bettas don't do well for long like that. The chances of an electrical malfunction in a heater are very remote. Just make sure you get an appropriately sized/powered heater and a good brand. The risk/benefit ratio on this point is huge. No brainer. Please listen to the advice about the cory. There are many, many good reasons for the advice. I'm so glad you posted here, your betta will be feeling much better soon I assure you!

 

I feel all the advice we can give is already well-explained in this thread. Like I said, we can't fix your fish for you - but it's good that you came here for advice. I'm not sure how posting about a problem and then insisting that there is no problem makes sense, but here we are - and that's okay, we're fellow fishkeepers here to answer your questions and share our knowledge! It's our hobby so that's what we do. But convincing you that the advice we give is solid isn't our hobby, nor is it our job, and at the end of the day ensuring your pet's well being isn't our responsibility - it is yours.

To summarize:
To feel better about creating an environment that made your fish sick you can insist you're doing everything right.
To get rid of the fin rot you need to get a heater and medicate your fish - which, yes, is definitely fin rot, yes really.
To prevent this problem form recurring, you need to up your water changes, reduce filtration, and get rid of the cory.

That's kind of all there is to it. Good luck!