Nice little filter. When I received it, the box listed it for up to 15 gallons. Even with it turned all the way down, the flow was too strong for my 5.3. It also makes a low rattle/hum when working so not the best option if you want something really quiet. It would probably be great for a 10 gallon though.
Fluval products never disappoint. This is a very powerful submersible filter a notch or 2 above the Zoo Med turtle 318 for small setups. What makes this unit nice is that the sponge filter chamber can be substituted with small bio-balls as a filter substrate. I'm using this for small juvenile single turtles and works great. Best of all, super easy to clean rather than the clumsy canister filters that are a pain to clean with all those small parts and tubing, etc.