Where Am I Going Wrong? 125 gallon freshwater tank

Up until yesterday I had 1 juvenile tiger Oscar for about three months and I still have 1 juvenile Pleco in a 125 gallon tank. The Oscar died yesterday. I dont know the cause and I'm hoping talking through here, some members can give me some ideas. I'm concerned to get another. I don't like being responsible for any loss of life and I'm pretty bummed out. Please offer some advice if able. Details below.

I have a 125 gallon freshwater tank. I have many fake plants for privacy, a small piece of driftwood, and a few hideaways. The substrate is a sand bed. Filtration is 3 AC110's. Temperature between 78-80. My Oscar seemed to respond to higher range. I do water changes twice per week, mostly because of my paranoia about sand beds and gas build up. I vacuum while the water change happens.i remove fake plants and hideaways in quadrants when I clean as they usually have debris under them. Those plants are rinsed and returned. I have two full length LED light strips but using them both produced algea quick. I'm down to one about 10 hours a day. I have two air stone strips. I have two power heads but haven't used them as much because they're a bit strong for juveniles.

I use AC110's instead of canisters because my aquarium is on a built-in without storage under it, it's just a tough spot for that type of filtration. Between only having HOB and sand, I thought frequent water changes would help counter.

I use Tetra 6 in 1 test strips and test frequently even though I think it's kind of foolish. Where I have two juvenile fish in a huge aquarium and water change twice a week, the values are always the same. The water source also tests the same:

Nitrate: 0 Nitrite: 0 Hardness: 75 Chlorine: 0 Alkalinity: 0 PH: 6.4

I was advised Oscars are tolerant of low PH and stability is more important than chasing a value.

The Oscar suddenly became more shy and refused to eat over a 7 day period before dying. 6 days of these symptoms until the 7th and final day, I found him nearly dead, colorless one side, cloudy eye. This appearance happened nearly overnight. Other side had more color and eye intact. It looked like scales had been worn on colorless side. Found Pleco... On him? Oscar died a few hours later. Pleco is eating and acting normal.

Oscar diet is freeze dried shrimp/mealworms, hikiri pellets, some bloodworms, brine.

Oscars natural quirkyness made it difficult to know what to treat, if anything, at first. By the time physical symptoms developed, cloudy eyes, scale damage, he was gone that day.

I could really use advice, suggestions.
Photos of set up and fish day of death.

 

I think you might be trying a bit too hard. Maybe one 25% - 50% weekly WC.
Although Oscar's look to be indestructible tanks, in reality they are prone to Hexamita and are often the topic of unexplained deaths.
You have a very good filtration system, but with such a low stocking level the filter media probably has a very low population of beneficial bacteria. A situation which makes the aquarium environment fragile & potentially unstable.
The low PH of your tap is very well suited for South American fish.
Since you apparently like bigger colorful fish, look at Severums or Geophagus. Big Tetras would also go well. Groups of Columbian, Buenos Aires & Bleeding Hearts should thrive in your tank.
Become active in the forums & hang in there.

 

Seems like you’ve got everything right in the tank. Though it does look more like an angelfish tank than an Oscar tank. Just guessing here but being a juvenile it may have needed company. They are usually kept alone when adults. Last time I had one I fed it minnows and than feeder goldfish. Got huge and lived about 10yrs.
your filtration is right for that tank but as A201 said because so few fish and such a large volume of water the bb is really spread thin. For an Oscar that small you may want to grow it out n a smaller tank. Get it to about 6+” before moving into the main tank. Next oscars like to move everything around in a tank. So I would do away with all the plastic plants. Another thing is oscars have a slime coat on them and the pleco may have discovered this and started eating it off the Oscar. The stress of this happening to it could also have killed it.

 

I think you might be trying a bit too hard. Maybe one 25% - 50% weekly WC.
Although Oscar's look to be indestructible tanks, in reality they are prone to Hexamita and are often the topic of unexplained deaths.
You have a very good filtration system, but with such a low stocking level the filter media probably has a very low population of beneficial bacteria. A situation which makes the aquarium environment fragile & potentially unstable.
The low PH of your tap is very well suited for South American fish.
Since you apparently like bigger colorful fish, look at Severums or Geophagus. Big Tetras would also go well. Groups of Columbian, Buenos Aires & Bleeding Hearts should thrive in your tank.
Become active in the forums & hang in there.

Thank you for your input and advice. You are right that I have a low stocking level and I didn't consider what this would do to beneficial bacteria. In my mind, I didn't want an aquarium larger than 125 gallons and I thought with a mature Oscar and a mature Sailfin Pleco, I may be pushing my limits as it were. Granted these fish are juveniles now but I'm more of a buy up front to avoid needless transitions later kind of person. Am I miscalculating the space needs and could potentially keep more fish with these two? Aside from sizing requirements dictating stock, I was also concerned about additional tank mates becoming Oscar food eventually. I really just enjoy the Oscar personality and appearance. Perhaps it's not realistic with my set up?

 

Look at some of the other big fish options such as a male Green Terror (Rivulatus) a female Red Terror (Festae) or a Texas Cichlid (Carpintis) all are colorful and more likely to do better with other medium sized Cichlids or big Barbs. Here a few pics of some I've kept.

 

Seems like you’ve got everything right in the tank. Though it does look more like an angelfish tank than an Oscar tank. Just guessing here but being a juvenile it may have needed company. They are usually kept alone when adults. Last time I had one I fed it minnows and than feeder goldfish. Got huge and lived about 10yrs.
your filtration is right for that tank but as A201 said because so few fish and such a large volume of water the bb is really spread thin. For an Oscar that small you may want to grow it out n a smaller tank. Get it to about 6+” before moving into the main tank. Next oscars like to move everything around in a tank. So I would do away with all the plastic plants. Another thing is oscars have a slime coat on them and the pleco may have discovered this and started eating it off the Oscar. The stress of this happening to it could also have killed it.

Thank you for your input/advice as well! I had the general feeling like I was doing all the right things but it was something I hadn't considered before.

I realize the plastic plants will need to go at some point but my juvenile oscar really seemed to enjoy the cover and would spend a lot of time... lurking behind them. It's a visual obstacle as an owner but something I figured the Oscar needed/felt more comfortable with- at least at this stage. What are your thoughts? Would feel just as comfortable without? Wait until they start being moved around to remove them? I had been considering getting some bigger driftwood and/or bonsai driftwood for a more natural look and to eliminate plants down the road. Would that be a better fit?

Now, you raise two great points, one of which I should have thought of:

When my Oscar appeared to be very near death (laying on bottom barely breathing), my pleco was on top of him. I could not see any meaningful motion. But it would certainly explain why one side of the Oscar looked raw. I can say I never witnessed any meaningful interaction between those two fish other than that moment. I have no doubt now that when the Oscar was compromised, this could have been what finished him. I just didn't know/didn't think about why this could be. Have you seen this behavior before? Do you feel this is more likely when an Oscar is weakened or do you think it may have been the cause of it all?

The other point you made about a solitary Oscar is something I wondered. I ask the LFS how they would do alone and was told it would be fine. I read online it's generally fine. But of course you generally always see them together in pet stores. Also, maybe having two would put a little distance between them and the pleco.

Full disclosure, I picked up two new juvenile Oscars yesterday. I am upset about my initial loss of course but I don't want to give up. If this doesn't work out, I will not get new ones. But I really enjoy them and I am going to try my best.

All of the above being said, do you have any other additional/supplemental advice/suggestions?

 

This probably has nothing to do with the oscars death but I see you are missing one of the most important tests. Unfortunately most test strips don't test for ammonia. I highly recommend you get an API Master Freshwater Test Kit. With it you can keep an eye on pH (both high and low), ammonia, nitrites and nitrates.

With a pH as low as you are seeing, low bio-load level and the twice weekly water water changes you wouldn't think ammonia would be a problem. It is possible you have ammonia in your tap water though and if so you are adding it to the tank each time you do a water change. Without testing for it we can't know whether you do or not.

 

This probably has nothing to do with the oscars death but I see you are missing one of the most important tests. Unfortunately most test strips don't test for ammonia. I highly recommend you get an API Master Freshwater Test Kit. With it you can keep an eye on pH (both high and low), ammonia, nitrites and nitrates.

With a pH as low as you are seeing, low bio-load level and the twice weekly water water changes you wouldn't think ammonia would be a problem. It is possible you have ammonia in your tap water though and if so you are adding it to the tank each time you do a water change. Without testing for it we can't know whether you do or not.

That's a great point- I haven't tested for ammonia and just assumed if my nitrites/nitrates were low or non-existent, I was safe. And I think you highlighted one of my weaknesses. I figure because I do twice a week water changes, I can get away with a lot. But not if the tap water is an issue.

These kits are not in stock at my LFS and the soonest I can get advised kit is Saturday (already ordered). I've ordered API Ammonia test strips for Wednesday so I can get a more immediate read and have a better kit for future use. I will report back here. Thanks for suggestion!

Look at some of the other big fish options such as a male Green Terror (Rivulatus) a female Red Terror (Festae) or a Texas Cichlid (Carpintis) all are colorful and more likely to do better with other medium sized Cichlids or big Barbs. Here a few pics of some I've kept.
View attachment 797493View attachment 797494View attachment 797495

You are quite right, they are big and beautiful! Is the behavior similar to Oscars? The sort of puppy-dog type fish?

Also, would you say these fish should be in lieu of Oscars or as additional tank mates?

I picked up two juvenile Oscars yesterday and introduced them to my tank. Still just the Oscars and the pleco. I don't want to stock my tank with a bunch of juveniles now and end up with overcrowding/eating each other later, which is why my stock is the way it has been. If you think there's room or benefit for more, I would like that- I just have a preference to keep the fewest fish possible to achieve space/happiness balance. If these Oscars do not make it, I will not buy anymore Oscars and will take a look at some of these other beautiful fish that may be more agreeable with my set up.

I appreciate you being the first to provide help and continuing to follow-up. I've had trouble getting advice on reddit, which has been my go-to for a number of other things. I know there's a lot of knowledgeable folks out there and I am trying to treat this situation as a learning opportunity. I don't want to lose another Oscar, or any fish.

 

Well what I know about plecos is when they find the slime coat on a fish they will go after it. Plecos can be vicious. Ive seen them go after discus and had to remove. But there where over 10 discus and it wasn’t just picking on one. And I’ve seen one tear apart a dying fish before I could do anything. So they have the ability to kill fish. Being a sailfin I would remove it and get a bushy nose.
Good that you didn’t give up and great advice your getting already.
The cichlids A201 is suggesting do not have the personality of an Oscar. Maybe a puffer fish has a similar personality. But none that I know have an oscars personality. Next those cichlids would be to aggressive for an Oscar. Oscars really aren’t that tough.
Being you got 2 more I’d get rid of the pleco and see how it goes. Yes test your tap also let the water sit for 24hrs and retest ph,gh and kh to see if they change.

 

This probably has nothing to do with the oscars death but I see you are missing one of the most important tests. Unfortunately most test strips don't test for ammonia. I highly recommend you get an API Master Freshwater Test Kit. With it you can keep an eye on pH (both high and low), ammonia, nitrites and nitrates.

With a pH as low as you are seeing, low bio-load level and the twice weekly water water changes you wouldn't think ammonia would be a problem. It is possible you have ammonia in your tap water though and if so you are adding it to the tank each time you do a water change. Without testing for it we can't know whether you do or not.

I performed three water tests: in aquarium, water from tap after a few hours, and water straight from the tap. It yielded the below results:

Aquarium and Water Sitting for a Few Hours were Identical:

Nitrate: 0 Nitrite: 0 Hardness: 75 Chlorine: 0 Alkalinity: 0 PH: 6.4 Ammonia: 0

Straight from tap was a little surprise in PH and KH:

Nitrate: 0 Nitrite: 0 Hardness: 75 Chlorine: 0 Alkalinity: 40 PH: 8 Ammonia: 0

No ammonia in any of the tests. What are thoughts for that PH/KH and the change over a few hours?

Well what I know about plecos is when they find the slime coat on a fish they will go after it. Plecos can be vicious. Ive seen them go after discus and had to remove. But there where over 10 discus and it wasn’t just picking on one. And I’ve seen one tear apart a dying fish before I could do anything. So they have the ability to kill fish. Being a sailfin I would remove it and get a bushy nose.
Good that you didn’t give up and great advice your getting already.
The cichlids A201 is suggesting do not have the personality of an Oscar. Maybe a puffer fish has a similar personality. But none that I know have an oscars personality. Next those cichlids would be to aggressive for an Oscar. Oscars really aren’t that tough.
Being you got 2 more I’d get rid of the pleco and see how it goes. Yes test your tap also let the water sit for 24hrs and retest ph,gh and kh to see if they change.

Thank you for elaborating on this point about Plecos. I believe some of the responsibility for interactions between the Pleco and the Oscar were my fault. My Oscar was pretty good aggressive and would often eat or "pick up" and drop Pleco wafers at feeding time, even if I would feed Oscar first. I wouldn't doubt if he went around picking at them later. That being said, I'm not sure the pelco was getting the food he needed and may have turned to/on the Oscar. Now that I know what can happen, I am trying to see if I can more directly feed the Pleco and see if I can keep them together. I feel a bit of guilt giving my Pleco back to LFS, especially if I contributed to this. Do you see any issue playing this by ear for now? The Pleco uses a hideaway frequently, so I've been putting his food in there and he seems to be getting it easily now. I still haven't witnessed ant negative interactions but I do wonder what happens at night. For now, both new Oscars greet me and seem in fine health.

I water tested aquarium water, tap water in a cup that sat for a few hours, and tap water directly. The only variation was in PH/KH in direct tap water. It seems this variation changes within a few hours. PH drops from 8 to 6.4 and KH drops from 40 to 0. All other values are the same across all sources and tests. Zero ammonia present in all tests. Is this problematic in any way?

To me, it seems the main suspect with the previous Oscar was the Pleco. Although I saw no damage at all until the day he died.

If you have any other advice, I'd like to hear it. If you think I should lose the Pleco regardless of the backstory, I'm inclined to do that.

 

I performed three water tests: in aquarium, water from tap after a few hours, and water straight from the tap. It yielded the below results:

Aquarium and Water Sitting for a Few Hours were Identical:

Nitrate: 0 Nitrite: 0 Hardness: 75 Chlorine: 0 Alkalinity: 0 PH: 6.4 Ammonia: 0

Straight from tap was a little surprise in PH and KH:

Nitrate: 0 Nitrite: 0 Hardness: 75 Chlorine: 0 Alkalinity: 40 PH: 8 Ammonia: 0

No ammonia in any of the tests. What are thoughts for that PH/KH and the change over a few hours?

It seems your tap water lacks the buffers needed to hold the pH and kh up. Your bacteria is struggling to grow with the pH setting this low in the tank. I will recommend you raise and stabilize the pH up close to your tap water the most natural way I know of to do it. Add some Crushed Coral to your filter. For a tank this big it is going to take quite a bit of it. I would put at least 2 cups of it in one or two media bags and put them in your filter. If there isn't enough room for that much put as much as you can and another media bag situated inside the tank but somewhere in the flow of water.

To me, it seems the main suspect with the previous Oscar was the Pleco. Although I saw no damage at all until the day he died.

If you have any other advice, I'd like to hear it. If you think I should lose the Pleco regardless of the backstory, I'm inclined to do that.

If your pleco is going after his tank mates if you can make sure he is well fed maybe it won't continue to happen. If he is in fact going after them it may continue being a problen though. Thankfully none of the plecos I've ever had has don't anything like this.

 

It seems your tap water lacks the buffers needed to hold the pH and kh up. Your bacteria is struggling to grow with the pH setting this low in the tank. I will recommend you raise and stabilize the pH up close to your tap water the most natural way I know of to do it. Add some Crushed Coral to your filter. For a tank this big it is going to take quite a bit of it. I would put at least 2 cups of it in one or two media bags and put them in your filter. If there isn't enough room for that much put as much as you can and another media bag situated inside the tank but somewhere in the flow of water.


If your pleco is going after his tank mates if you can make sure he is well fed maybe it won't continue to happen. If he is in fact going after them it may continue being a problen though. Thankfully none of the plecos I've ever had has don't anything like this.

As an update to this thread, there have been a few developments that I hope help some others. Firstly, I got two juvenile Oscars shortly after my first one died. One of those juveniles quickly seemed to be exhibiting some of the same behavior as the other one before it died). This had me panicking. I monitored for another couple of days and noticed what I felt was damage to slime coat and general lethargy, I was increasingly concerned this fish was going to die in the next few days. I used Stress Coat + and noticed an immediate improvement in just 24 hours.

In addition to this, I bought crushed coral on Amazon. I put it in a media bag and exchanged it with the crushed coral that came with my AC110's a few days ago (ineffective is the best way to describe the crushed coral that came with the filters). Overnight, my PH went from roughly 6 to 7.4 where it has been steady since.

I did a 30% water change and cleaning last night. I used Stress Coat +. This morning, I tested the PH of the water and it was 7.4. I decided to test the PH of my tap again and it was 7.4. It seems the coral is holding the PH, which is great. The Stress Coat + has improved mood and slime coat to a noticeable extent; particularly with the one ailing Oscar. I am wondering at this point if my PH is better if I need Stress Coat during every water change? All my other parameters ammonia, chlorine, nitrite/nitrates all zero. But I'd hate to put them back in a position where they're ailing again just because I am feeling frugal.

My pleco spends a lot of time in a cave in the corner of the aquarium. My Oscars have no interest in this cave, so I've been putting Pleco food directly inside there to ensure there is no competition for his food. This system has me feeling better and I have not noticed anything unusual or that would lead me to suspect the Oscars and Pleco are having any negative interactions.

I have eliminated a lot of fake plants, leaving only the biggest ones mostly to simplify cleaning.

Overall, I think I had a few potential factors I wasn't getting right that helped me turn this situation around:

1) A201 brought to my attention that I should observe slime coat more. Stress Coat + does have properties that allegedly help repair slime coat. When one of my Oscars was sick, this did help immediately. This same advise also caused me to re-think how i was feeding Oscar/Pleco to ensure there would be less competition for food and thus fewer interactions with each other.

2) Mattgirl brought to my attention that my PH instability could be playing a role in all of this. The suggestion of crushed coral was a simple, natural solution that was inexpensive and the result was also immediate.

3) Admittedly, I wasn't doing a great job checking water temperature from tap during water changes. I assumed if I was reasonably close, the amount of water I was changing wouldn't impact the overall temperature much. But I also noticed my mop sink tap has a tendency to change temperatures without my adjustment. I have been monitoring temperature a lot more during changes to ensure more consistency.

These few things have made an immediate, noticeable change and I wanted to share my appreciation for the members that offered their advice to me. Thank you! I hope this helps someone else down the road!

As a general observation, my two Oscars (maybe 2" in length) are a little fussier with food than I expected. They act very hungry, approach me for food. They will often refuse pellets, shrimp, blood worms, etc. They will eat things and spit them out. They DO eat, it just takes either persistence to find the thing they will eat OR I will skip a feeding and they're a little less picky the next time. But they just seem fussy for Oscars. Does anyone have any feeding suggestions? Particular brands or foods they have more success with? At this point, I feed once in the morning before work and once before lights out when I return home from work. I try not to feed more than they will eat while I'm standing there for a few minutes. But where they get fussy, I end up with three or four different types of food in the water sometimes...

Thanks again to this forum and its members!

 

Thank you for the update. I am happy to read things are now leveling out for your water pets. That is a good recommendation for the Stress Coat +. Before you started adding it which water conditioner were you using? Even way back when we were still using our deep well I still added my water conditioner with each water change. I considered it a very inexpensive form of insurance. Back then and for many years I used AquaSafe. I now use Prime. I like that it takes so little of it to get the job done.

Paying close attention to the temp of the water is important. As long as it is within a degree or two it should be close enough.

 

My question is we’re the stress coat and the crushed coral added at the same time? Next to all do aquaclear filters come with crushed coral? Just to make it clear crushed coral is white in color .

What test kit are you using? 8 ammonia means the kits not working right. You are using both bottles? Next no nitrate again are you using both bottles and shaking them like crazy?

Blood worms are not the best food for oscars. They’re a treat at best. What type of shrimp are you feeding and pellets? They need food with high protein.