Skimmers

Skimmers generally aren't needed for people with average water quality and average fish. I couldn't tell you exactly which fish does best with these devices seeings how I never use them but I can tell you that unless your an advanced aquarist with a complicated situation, you probably don't need them.

Skimmers--if you mean protien skimmers--work by forcing tiny bubbles through the water column. Protiens collect on the surface of the bubbles, and then collect at the surface when the bubbles break. For FW, they don't really work as well--the surface tension of FW make sit hard to get the tiny bubbles needed. They will help, but not as effectively as in SW. And, water changes really will help more then a skimmer.