I've used it in my tank 3 times a couple of months apart. The first time it worked like a charm. The second time it did not do such a good job, and one of my fish died. The third time it did nothing and another fish died. I'm not sure if the fish isdieing as a direct result of the chemi-clean, or if it is a result of the cyano reacting to it . Anyway, the guy at the LFS told me that the cyano can develop some tolerance to it if you keep using it. I guess that is what happened.