30 gallon FOWLR

[jjen8400]

I recently took over a friends 30 gallon fowlr setup she was moving out of state. It has a lawnmower blenny, clownfish, a couple of different types of damselfish (which I have read are not compatible) and a yellow tang (also read this should not be in such a small tank) but I noticed most people talk about some sort of cleaning crew. And I'm not sure what I need the substrate is cc. open to any suggestions please help I have noticed some sort of dark colored buildup starting on the cc

 

[saxman]

Just so ya know...clownfish are also nasty little damsels in fancy PJ's, but most people overlook it because they're "cute", but I digress.
Yes, the tank is less than half the minimum size for a yellow tang, so it should be re-homed. Tangs are nervous, pacy fish in small setups and often get nasty once "cabin fever" sets in.
The dark stuff you see on the substrate is likely a diatom bloom because the tank is going thru a "mini cycle" from being moved and set up again. It will get worse, but it will also go away all on its own once the tank stabilizes a bit.
CC will need to be vacuumed regularly to remove all the junk that gets trapped between the grains. A better choice would be #3 or #1 grade aragonite. It isn't as coarse as CC, and doesn't trap detritus and uneaten food, but it "lays down" better than sugar-fine aragonite sand.
Yes, a CUC is very beneficial. Here's a good primer on selecting and stocking a basic CUC:
http://www.lionfishlair.com/cuc/cuc.shtml
HTH