There is a pinned topic on this somewhere. Basically, it's going to be bad news for any scaleless fish and likely to be unhealthy in the longterm for any freshwater fish unless specially adapted for brackish conditions (like mollies which can cope with both).
Adding it as a tonic is a bit like eating antiobiotics all the time; you'd be better off leading a healthy lifestyle and developing your immune system. Healthy lifestyle for fish= varied diet, no overstocking and regular water changes.
