I keep my frog tank next to my fry tanks and they have a sponge filter powered by an airpump. Be careful of the type of filter you use as they can get their legs trapped in them. They also don't like a lot of current.
I have to differ with SirMinion on this. Aquatic frogs are an exception to the general rule and do better without filtration. The problem with filtering their tank water is that the vibrations from the pump are disturbing and stressful to them.
You could keep them uncycled by emptying and washing everything in their tanks at least once a week, or by cleaning the gravel and doing weekly big water changes which will give you some beneficial bacteria living on the gravel. I prefer the second option.