How Have My Guppies Got Fin Rot

Hi,I have had my tank running for about 4 months and everything's been going good apart from noticing my tetra are getting fin rot.I put 2 female Mollies in and my 3 of my male guppies wouldn't leave 1 of them alone,I woke up to a dead guppy with what looked like a big chunk of his underside had been bitten off also 2 other guppies tails had been bit.I rehomed the Mollies after 3 days then noticed the fin rot starting on the tetra.Could this be stress off the tank being disrupted? I do a weekly 20% water change but when I got the water checked at aquarium shop he said the nitrates were a little high and to do an extra water change.Also been reading it could be fungal or bacteria affecting tetra? Any advice would be great
Thanks

 

You could have brought a form of fin rot in on the Mollies, and then had the high nitrates give it an ideal chance to spread. I'd up the water changes to 30-40% weekly, and look at possible overfeeding.
The mollies probably hit out at the guppies harassing them.

90% of the fin rot discussed here probably isn't fin rot. I'd be curious about a description and what kinds of tetras you have.

 

HI yeah their neon tetra,the ends of their tail fin looked a little white and now it looks like part of its tail is shrinking .Just thinking 1 tetra has pop eye could this be affecting the others?
Thanks

 

It adds up to a serious water quality problem. Pop eye is a symptom, raggedy fins are too. I would start an increased weekly water change routine, as soon as you finish reading this.
I don't use antibiotics in tanks, so others may have suggestions. Prevention, with careful stocking, light feeding and weekly water changes (without fail) are my approach. In your case, the cat's out of the bag.

 

Ah right I thought I was doing to many water changes just once a week and you get told different things.Would increasing water changes to 3 times a week changing 20% be ok? Sorry to ramble on just want to get it right
Thanks

 

Greatly increased water changes and a wide spectrum Med like Seachem Paraguard would be a good starting point. It will be a lot of work but if I had a trainwreck going on my plan would be something like this...

Now- Massive WC, no less than 75% and Med the tank full dose.
-Tomorrow- 50% WC and Med again.
-For the next few days a 25%+ WC and Med again.
-After 3 or 4 days you will know if it's working or not.
-Daily salt baths on the side for species that are salt tolerant. That can be magical sometimes.

 

HI thankyou I will start water changes tonight but won't be able to get meds till Thursday, at work till late tomm.
Thankyou for your advice really appreciate it.

 

HI thankyou I will start water changes tonight but won't be able to get meds till Thursday, at work till late tomm.
Thankyou for your advice really appreciate it.

Sounds good, but two days is a long time so get some salt as it's usually better than nothing. Medicating salt doesn't have to be official aquarium salt. Just not iodized table salt or any other additives. Aquarium salt works, canning salt works, most rock salts work. Sodium Chloride is sodium chloride if nothing else harmful to fish is added. I'm not a huge fan of salting my main tank but it's better than watching fish die for two more days. Tablespoon per 5 gallons of tank. A salt bath is a healthy tablespoon in a one gallon bowl. Guppies and other livebearers for an hour. Other fish watch closely and if they freak out end it. Salt shakes off many parasites fast. It is also an irritant and stimulates the fish's slime coat production if I didn't mention that already.

 

I will and hopefully see a change over the next few days.
Thank again