Was anything used to clean the tank besides cold water? Maybe a residue or something like that.
It sounds like high ammonia, but you said water checks out. You may try draining it completely and refilling. Otherwise if it is some type of very aggressive parasite you may need to nuke the tank and start over.
It sounds like high ammonia, but you said water checks out. You may try draining it completely and refilling. Otherwise if it is some type of very aggressive parasite you may need to nuke the tank and start over.