The experiment aim is to study how certain factors (the independent variables) affect the concentration of dissolved oxygen, which is the dependent variable.
In this case, the independent variable, which is the one the scientist or investigator set the values of interest, is the number of fish and Elodea plants in the aquarium. Other independent variable is the presence or absence of light as this condition is also set by the investigator and it it posible to study the effect of it on the dissolved oxygen.
