Fin rot, stress or nipping?

My brother just got 2 platies and put them in his 25 gal tank. Its cycled and no other fish. He said water tests were good. He brought them home from the pet shop and did the float in bag, then added some aquarium water to them in a bucket, then scooped them out into his tank. He said they seemed fine, eating and swimming around, but the next day one of them had lost his tail! Half was off and half dangling by a thread. Then it completely fell off! I told him maybe the other one bit it off but he hasn't noticed any aggression between them. So what the heck makes a platy's tail fall off in a couple of hours?? I can't see fin rot bc his water is great. Freak accident, injury, stress from a new tank?? Told him I'd ask for him since I'm part if this forum.

 

Any decorations or fake plants with sharp edges in the tank?

 

Not really..not sharp to us but how delicate are they?

Update, he days yesterday and today its not interested in food.

Says*

 

Fish is still not eating. Sad no one is answering.

 

It's difficult to provide answers with the small amount of details you have given. You say the water is good, but without the actual numbers from a test kit, it's hard to agree or disagree if the water is good.

Without a photo of the tank then it's hard to say if anything in there has caused the problem.

 

The males can be aggressive with each other. They're best kept in small groups so it could be that there was some fighting....It's always better to get as many as possible (obviously depending on tank size and other inhabitants in the tank, plants or not, etc). You will always get more typical behavior and less stress when you add most fish in groups.

 

I tested it for him with my kit. 1ppm ammonia, .5ppm nitrite, 10ppm nitrate. Don’t have a photo, I’ll see if he can text me one. Plastic plants and a plastic molded coral piece on the bottom, plus a hidey cave. Small gravel substrate.

 

A cycled tank should have 0 Ammonia, 0 Nitrite and some Nitrates. It's possible that the addition of fish as knocked the cycle out a bit, but if it was cycled then the 2 fish shouldn't have caused an issue imo.

Definitely needs a water change to bring the ammonia and nitrite down.

 

Right, I don't think it was fully cycled when the fish were added. From what I've read, in the final stage of cycling those 3 values drop. Attached a pic of the fish.