A classic type of experiment with 2 identical tanks, and just change one variable between them and compare results at different time intervuls. (makes a nice line graph!)
One applicable to a real world situation would be more meaningfull. such as add simulated farm runoff to one and pure water in the other. I would hypothesize that the one with "farm runoff" ie. nutrients. will grow faster.
Ooh, or a fun one with one tank would be put all native plants to your state (wild collected) and one on your states invasive sp. list (SnS!) all in one tank. Interspecific compitition between natives and non-natives would be fun!
I could go on for days, this is kind of what im doing at work and in my free time, But for fish populations.