What are my levels of independent variable?

I'm doing research on closed aquatic biospheres and my sources are plants, water fleas, and snails. My independent variable is the amount of plants i put in the aquatic biosphere, so how please help my understand and find my levels of independent variable.What are my levels of independent variable?
That depends on what you're trying to show. If they idea is that plants generate oxygen, then you have to look up how much oxygen a plant of the size of the ones in your biosphere produce. Then you need to know how much oxygen your fleas and snails need to survive. Then you might establish 3 levels of plants- the numbers that might produce enough oxygen with minimal investment in plants, a mid-level, and then a maximum number of plants that you're sure would produce enough oxygen, but take up more room in the biosphere than is desired. Then you measure how your animals do in each condition.