You might want to look into a hillstream loach. They stay very small, only about 2.5", and are wonderful algea eaters. And they just look really cool. I have been keeping one in a tropical tank but am considering setting up a coldwater tank to move it over to because my most recent research suggests they would thrive better in that environment. They do need increased oxygen and current in the tank so you'll want to run air stones or a bubble wall. I have mine in a 29 gal and I run 2 airstones, one at each side of the tank, and that seems to do well for him. If you had a larger tank, I'd run more air. They are really awesome. Commonly sold as a butterfly pleco.