Plants in aquariums help to filter the water, get rid of ammonia, nitrate, and nitrites which are a result of your fish waste, and remove carbon dioxide and produce oxygen for the fish.

Since your project would involve live animals you will want to make sure that you don't actually do anything cruel to them so keep their tanks clean as you normally would if they were your pets (please keep them as pets after your experiment or give them away to good homes!!) When scientists use live animals (including humans) in their experiments, they are subjected to very strict rules. Mostly you want to limit any undue suffering of the animal and be humane to them.

You could have two fish tanks, one with plants (go to the local pet store and ask about good aquatic plants) and one without. Feed your fish the same thing, same amounts. Right before you clean the tank measure the amount of ammonia (and nitrate and nitrite) in the water (you should be able to get test strips from the pet store). You could also compare the abilities of different plants to clean the water (e.g. compare a household plant to an aquatic plant). I'm really not sure how sensitive these testing strips are so you may or may not get a huge difference. If you can you'll want to have at least 2 test tanks for each experimental variable (e.g. 2 tanks with plants and 2 tanks without), so you'll have a replicate.

You may need to try a few experimental set ups. Try taking a water sample every day instead of just right before you clean the tank. I read that you should clean the tank if your ammonia test reads anything above zero. You may need to play around with how you do the experiment a bit but always keep in mind the well being of your fish.

A link to some information about choosing aquatic plants:

Good luck and don't forget to be nice to your fish!