I am not a "marine" aquarist, I have freshwater. but what I do know is that these things will filter out diatoms, and parasites like Ich and flukes. They also give the water a "polished" look. I will admit that they do cost. you may try an online auction site for a used one.
It's a waste of money IMO. Diatoms go away after a couple weeks. Just like uv that type of gadget is more a fw thing and even in fw I wouldn't waste the money on that piece of equipment. I'd rather buy other things that are more crucial.
20 Gallon SPS reef
29 Gallon FW "fat catfish" tank
20 Gallon Long Dart Frogs "Zig" and "Zag"