When I had a small pico 2 1/2 gallon tank, we retrofitted it with a reptile filter. We filled it with the live rock rubble. Was like a mini refuge in there, minus the macro. Kind of like a minature canister really. It worked GREAT. Little too much power for the 2 1/2 gallon tank, but the zoas were fine. For a five gallon, probably work alot better. The plus size is it doesnt take up alot of your room inside the tank, it just sticks to the backglass.

This is very similar to what we used. http://www.petco.com/product/14971/Z...px#description