I had a prizm when I first got into the hobby (wasn't I so smart to avoid the sea clone! Ha yea right). Ran it on the back of my 55 before upgrading to a deltec
It was a pain to calibrate, it would overflow and run the skimmate back into the tank, and it was obnoxiously loud. I mean, I could hear it from another room, which wasn't always bad, cuz after a few weeks I could tell the sound of when the collection cup was full.
I would def skip the prizm and go with a deltec, heck or even Coralife Superskimmer. I'm running a CSS 65 on my 20, and its pulling out waaayyyyy more then my Deltec did on my 55.