A while back I experimented with making a Walstad jarrarium. I found a square, 2.5 gallon jar that was a great desktop size. I lined it with about an inch of organic potting soil and capped with a half inch of black, coarse sand. In the center was a vaguely tree-stump shaped piece of drift wood approximately 6 inches long. I wrapped the “tree trunk” with java moss, and the surface was coated with Amazon frogbit. Lastly I planted around the jar with java fern and hair grass. I had a small air pump slowly bubbling to help keep the water circulating.
For fauna I put in a few wild ramshorn snails. After waiting three months for the jar to stabilize, I added (as the title suggests) two male dario dario. I was worried they might be too aggressive, and there was a lot of tussling at first, but when the plants all began to grow (and they did, quickly!) they soon lost direct sight of each other most of the time and things settle down.
The most exciting part of this set up is that I never fed the dario dario a single bite of food. They lived completely on the NUMEROUS cyclops and worms that were thriving in the jar.
I would love to try and recreate a similar set up in the future.