How about fish tank pollution? How long of not changing the water before the fish up and dies? One goldfish, one bowl, simple and easy to perform!
I think this as I didn't get this done today at my house. Tomorrow, tomorrow, I love ya tomorrow, sigh.
Oh, and how about the growth rates between houseplants that you water with used fish tank water, to houseplants that you water using fresh tap water? In case you don't want to make a fish sad and depressed.