Aquaclear makes a good series of external power filters. Takes up less space in the actual tank. I wouldn't get the smallest though, get the one that's rated one up. I think it's the Aquaclear 30 rather than the Aquaclear 20. If you have it at maximum flow, you won't be over-filtering, but if you have to reduce the current for any reason, you won't be underfiltering. They are also pretty affordable.
llj