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.
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20 Gallon SPS reef
29 Gallon FW "fat catfish" tank
20 Gallon Long Dart Frogs "Zig" and "Zag"