About as simple as you can get. Empty 20 oz. bottle, airstone, some used fishtank gravel, and a couple sprigs of hornwort. I'm working on a larger main tank for them right now. Basically the same thing only bigger, and with a sponge-filter.
This one will be about 3 gallons, and the filter is the same concept as the one I made for the Betta tank, only on a smaller scale (I only used the bottom PVC piece and the grate, with a single piece of pipe running straight out. I also filled it with 100% API filter-sponge).