So a friend of mine had a betta in about a 5 gallon heated (82 F) filtered tank. One day we noticed his anal fin and dorsal fin looked like someone had grabbed them from the tip and started to twist or roll them up toward his body. The next day they were twisted or rolled all the way up to his body and he was dead.
I have not had bettas that long but have had other fish and never seen anything like this happen to fins before. I have seen fish in pretty bad shape but they never went from swimming/eating as normal to dead the next day either.
Her water was dechlorinated, treated for metals, the tank was cycled and the readings were Amonia 0, NitrItes 0, Nitrates <10 (dropper tests day before he died) Any ideas??