changed from testing the effect of pH on duckweed growth to testing the effect of light levels on duckweed growth
Explanation:
There are at least two variables in every experiment. The independent variable and the dependent variable. The independent variable is manipulated and its effect on the dependent variable is evaluated.
pH and light levels are both independent variables. When one switches from testing the effect of one of them on duckweed growth to testing the effect of the other, then a change in independent variable has occurred.