skimmers arent filters in the sense of breaking down amonia to less harmful forms. That would be live rock. A skimmer just pulls out crap in the water.
But i strongly suggest the aqua c remora. I've got one on my 29 and its awsome. The cpr might be ok. Lots of people say they're just ok, nothing better, and lots say they're not so great. But it seems the key is to have them on a small tank like yours.
Another one to look at is the coralife superskimmer 65. Many people like this skimmer due to cheap price and "good" quality. I personally hated mine tho. Never did get it to work right so its just sitting there now.