gotta agree with the others, i have a turbo twist 9/w and its mounted close to my sump, not on the back of the tank, but i will say, it will not harm anything in you tank, it will help everything...wont hurt live rock (how ?) dont get that one, it will kill unwanted parasites definately,,,and i use a powerhead to run it also, It's probably the best investment i have purchased so far , and talk about crystal clear water !! my fish look like their floating in air , its great when everyone asks how i keep my water so clear !! i bought the recommended size for my tank 125 gal, which requires the 9/w but you can use a larger one if you like, whats important is the dwell time passing through the light, you want it to do its job so dont over pump the water through it , give it enough time to kill whats passing through it, so I recommend it by all means.. good luck.