The whole idea of a mini biosphere is ridiculous. It doesn't teach children anything about the world, and instead instills the idea that small animals are disposable objects that don't feel pain or need to be regarded with any degree of respect. I don't understand the value of this "experiment" at all. Studying existing environments would be much more beneficial. Trying to shoddily replicate them without understanding of how delicate and complex these systems are in nature is irresponsible and useless. The students don't know anything about the origin of Endler's livebearers, they don't know about their social and spatial needs, they aren't taught about the nitrogen cycle and how impossible it would be to maintain in an unfiltered, unaerated tiny container. Sure, plants will help, and depending on the animal you get, it may be able to feed on decaying plant bits and green water--but really, a pickle jar is no place for a fish, and it doesn't take a scientist to figure that out.
There's a special place in hell for whatever "educator" dreamed this one up. If it were me, instead of doing the project, I would write up a detailed report on how terribly this exercise falls short and betrays the entire point of a biology class. That's me, though.