I have contemplated on this a lot.

You could run at least 4 inches of water and drill your enclosure to allow the use of a canister filter to handle the waste products.
You could also keep cleaner fish in the water like emerald cory's and similar fish. They normally stay on the bottom so the frogs
wouldn't have much of a chance of catching them.
With a good canister filter like a fluval 405 you can use it to make a water fall also.

Also you need to have places to haul out if the frog lands in the water.
these guys are clumsy some times.
I wouldn't use a plecostomus since they have very sharp fins and they can lock them and really do dome damage to a frog.
You can get some guppies and the frogs might get lucky with those and they are not bad for the frog.
I would setup the enclosure first and then add the fish and let the water cycle and get stable for at least a month before adding
frogs.