Aquarium Fish Tails Being Bit Off Does My Male Guppy Have Fin Rot?

Does my male guppy have fin rot? - aquarium fish tails being bit off

Recently one of my guppies had a piece of his tail by little, I think I was, until 1 bitten, I have many male fish in my tank down to have fun together and 2 only, as it was air. But now I think he developed the decay of the tail fin, it is gone, and it's not even when she was little. He can rot or the tail was all the time? I have to use more water changes and aquarium salt with every water change. Also have done water changes are always completely changed. How should I treat that I had the tank for the storage of everything I try to isolate them and treat, treat? It has also in the tank for some time and not a new fish