How it works
To succeed in making a complete ecosystem you need to have something that is alive like fish, you also need something that will keep them alive such as oxygen and food. And of course some sort of hygiene keeper, like a snail.
We took two comet goldfish as our living species and we added a cabomba plant to provide food and oxygen. The cabomba plant is not very fibrous so it's easy to digest. It also gives out an inordinate amount of oxygen.
We have a ramshorn snail that keeps the tank clean. He eats algae, which if it's left too long to develop, it can grow bacteria's that can be fatal. The snail also eats left over food and the fish feces.
When you put it all together voila! An entire ecosystem!