Scientific Method Gear Experiment

Imagine a pond with fish in it. Students will turn the gear to increase the temperature of the pond. This will affect the population of fish. Once the temp/oxygen is out of range for the fish, the population will drop. There is a blank one so you can add whatever factors you want. Have them graph the numbers. You can use another one that the students don't touch for a control group. This is one print. It is best to turn the first and third wheel at the same time instead of just turning one wheel.
How did it go in class when you tried this? I am curious. Got better ideas?