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.
How did it go in class when you tried this? I am curious. Got better ideas?