Yesterday they were fine. Established tank. In it I have 2 blue acais both are dead now. 2 convicts still alive. I lost 4 mooris I think that's what there called they also died. All this happened within an hour.my parameters are as follows. pH 7.0 ammonia is 0 nitrite is 0 can't test nitrates. Tank is a 30 gallon. They don't have any diseases or look sick. What could be happening? I did do a water change yesterday and everything was fine after that. Any ideas of what could be happening?