Back in October we brought in several water plants from our outdoor pond to winter over indoors and floated them in a couple gallons of water. We recently took a look inside to see what kind of gunk was rotting away, but instead of finding a smelly mess, the water was clean, and full of living things. Snails were cleaning house, and small crustaceans of various types and sizes were swimming everywhere. Try a gallon jar of pond water with a plant or two in your own room, and see what develops over the next few months.