To test whether acidification would inhibit growth of Elodea, you would need several containers of the normal pond water in which Elodea grows. One would serve as the control and would not be altered. The others would receive progressively more acidification, and Elodea growth would be measured in each to see the effect of the acid.
Work Step by Step
Here the dependent variable is the growth of the Elodea, and the independent variable is the amount of acid, the control being no extra acid in the pond water.