To create an aquatic ecosystem that is self sufficient, there should be a fish and a plant.
We generally make fresh water ecosystem for which we first need ponds or river water which must contain some weeds or plants. Otherwise we should put the aquatic weeds in it. The water also contains algae. Then we need a fish to live in it and keep it in light.
The aquatic weed and algae make oxygen which is necessary for the fish. So that the ecosystem can be self sufficient.