I bought a set with various ostracods (clam shrimp, seed shrimpa and scuds), daphnia and cyclops shrimp, amphipods, ramshorn and lesser pond snails and either trumpet or trapdoor snail. It is closed up and is supposed to be a ecosystem.

1st food chain: The crustaceans eat plants and excrete which feeds the plants and bacteria.
2nd food chain: Bacteria are eaten by crustaceans which excrete and feed the bacteria.
3rd food chain: Snails eat plants, excretions and dead crustaceans and feed the plants.