:x i have a 65 gallon tall and i i let it cycle for 3 weeks and tested it and the water was perfect so i added 6 rainbows to start off the next morning the were all dead. i re tested and the water was still good i returned the fish and did a 60% water change and the replacements also died, with in 3 hours. my plecos were fine but every other fish that hit the tank they died. not just rainbows but geophagus and danios.... the water was also tested at the pet store and i was told it was the best sample he had in the last year. there was no visible disease and no chemicals were used to clean out the tank either. so does anyone have any idea what could have caused this???? nitrates nitrites and ammonia were all at zero