i had the same problem in my old 30 gallon tank, we went for months , we lost fish and everything, i tested the water over and over again, nothing, so i took it into my LFS and he tested and told me i had really high posphate levels. come to find out the rock that i had gotten was letting this stuff into the water, and if thats not the case then just get a posphate remover, they work, now i have a 75 gallon tank and i put this stuff in religiously, and i haven't had any trouble, and with the posphate i was getting green messy alge. just some thoughts