I have 2 of the little Azoo Filters. They say they are for 3 gallon tanks. I've had one running on a 5.5 gal for a couple months now that have some Red cherry shrimp in the tank and before that housed guppy fry. I have a small sponge pre-filter on the intake (keep babies from being sucked up) and it has worked well. The bio-load though IMO is not real large so it has been enough to filter this tank. It is crystal clear.