Have you tried rubber-banding an extra filter cartridge over the output part of it?
I did that with my 5 gallon froggie tank since they don't like strong current and it seems absolutely perfect for them. :) I think I actually got that advice on this site, now that I try to remember where I heard it... or maybe that was the pantyhose suggestion... which also worked, but that pantyhose over the filter gets pretty gross quickly, so I think the extra cartridge is better.
Robert Falcon Scott - Red salamander HM plakat
Passed on- Friedrich "Freedy"
Four African Dwarf Frogs - Dorkus, Sporkus, Baron(ess), and Bubbles!