If you're going to do a gravity drop through the filter from a low energy pump like an aquarium type, coffee filters might just be the thing. I would suggest the culligan, et al. type cylindrical domestic water supply filters, but I think that would require a fair amount of pressure - maybe so much that the inlet would suck fish as well as dirty water.
I foresee an aquarium filter pump, dumping water into a used Mr. Coffee (each $5 at a yard sale) that not only filters but heats the water to the optimum temp as well.
