Ecosystem project help?

I have to create an ecosystemusing Five two liter bottles and in the ecosystem I need some sort of aquatic animal but we cannot give them food besides he plant that we have put in the water and things that fall from the decomposition level above it we also can't change the water....it has to live for three weeks....whAt kind of animal can I use? Also the animals primary food source in it's naturalhabitat must be plants.....