they are designed to kill so thats what they do. there are some factors like flow rate and dwell time that play a part in it. the logical assumption would be that it would take longer to kill larger critters but in the end i think they would all be doomed after a while.... if you used it to kill alge and things of that nature only a couple of times a month then you should be fine, the same as using ozone to polish the water, but this is my opinion and no way fact.
My tank is about 4 months old, and so far it has run 24/7. I have a ton of pods, and have not had 1 sick fish. Of course 4 months is by no mean long term. I do have a powder blue tang and a sailfin both are fine... Never had ich..
