How long have you had it? Are you sure you're just not going through your case of the "uglies"? I have a Biocube 29 with a maroon clown, a green mandarin, 2 mini maxis and a couple brittle stars, and a snotload of snails.. I've gone through the diatom bloom, and now occasionally get small patches of red hair algae on my sand where there's not much flow.
Because of the mandarin, I'm guilty of WAY over feeding frozen foods, so make up for it with weekly water changes of 10%.
On the smaller tanks, you don't need a skimmer - you're your best clean up crew.