For fish keeping, it is a quite special case. Because fish tanks require well established good bacteria colony to complete nitrogen cycle to convert the ammonia produced by fish into less harmful nitrate, we avoid to "disinfect" fish tanks under usual circumstances. If we accidentally kill those good bacteria, the cycle will crush, and ammonia will build up to kill the fish.
In the cases of entire tank of fish are already wiped out by some kind of parasite or diseases, it is said bleach can be used to clean the tank, but it has to be 100% washed off before the tank can be used again. Of course, the tank will have to be cycled again in order to receive new fish.
For typical cases of bacteria caused diseases such as mouth rot on fish, we use anti-biotic. The ones which are harmless against good bacteria.
The last time I treated fish with mouth rot I used Metronidazole, Kanaplex, Triple Sulfa. I forgot which one actually worked since it was a long time ago.
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