I would see how you get on with either an airstone (with enough bubbles to 'break' the surface, but not enough to cause turbulence) or a box-filter, the sort that runs on an air line, if you want to use a filter.
Personally, I go for the "
no-filter, heavily-planted, keep-an-eye-on-the-water-quality" approach. I combine siphoning dirt out with frequent small water changes, and it seems to work. My tanks are fairly understocked (as far as newts/water ratio), and the plants play a huge role in keeping the water healthy.
Your
waltl are only small, so even though they'll create a lot of 'poop' when they're bigger, you'll be fine without a filter.
Remember to check the water quality (Ammonia,nitrites,nitrates) regularly.
Good luck with the planting, and post some photos

:happy: