Safe filtration for underwater frogs?

Is there any safe form of filtration for dwarf underwater frogs? I've heard that they will often get their legs stuck in normal filters and hurt themselves, but cleaning the tank out three times a week is quite a chore, is no doubt putting a lot of stress on the tank seams and no doubt quite stressful for the froggies...is there anything I can use for them? Would a biological filter work with frogs?