Can you give me a purpose and hypothesis for doing a closed ecosystem like a jar that has fish, snails and plants?
This question is interesting. I'll try my hand at suggesting some possibilities, but I'm no scientist!
I'm not sure if you have in mind a jar that's sealed shut and filled entirely with water or not, but I'm guessing that you do.
The purpose could be to determine if the plants and the animals are able to produce enough of what the other will need to survive: plants need carbon dioxide and animals need oxygen.
The hypothesis could be that the animals will produce carbon dioxide and the plants will produce oxygen, but they will probably not produce the necessary amounts for all of the life to be sustained. I suspect that, if the jar is sealed to prevent gases from entering or leaving, the oxygen levels will drop too low and the animals will die.
If the jar isn't sealed, you might consider whether the plants, snails, and fish will produce enough "food" for each other.
I'm not sure if I'm correct, but I think that there are at least two problems or complications with this experiment:
The ecosystem will not be entirely closed, in the sense that it will be fueled at least in part by direct or indirect sunlight. Without sunlight the plants would die, I think.
The fish will also need more than regular (dechlorinated) water to survive. If you've ever had a fish tank, you'll know that the ammonia-nitrite-nitrate levels have to be controlled through the cultivation of certain types of bacteria. I don't think that the fish will live very long otherwise.