It is the best choice for sensitive fish. I have used it on a dogface puffer without issue. Others have used it on angels, eels, etc. However, with sensitive fish, you really should very slowly ramp up the dosage to the treatment level. I would suggest that you slowly ramp up to treatment level over a 2 to 3 day period. Also, you want to keep you level on the lower range of .3-.4 and not .5. This may require a bit longer treatment period, but your fish will tolerate the treatment better.