Pistia or water lettuce is super easy to grow! It does really well and multiplies fast under high light and nutrients, but even in low light it will grow. As long as you don't have large fish, the fish shouldn't eat it. However, my koi love to much on them so if you have big fish watch out! In the aquarium there is a small variety of Pistia as compared to the pistia stratiotes, a common pond plant, found at water garden centers. Regardless, they are both easy to grow. As for CO2, I'd imagine the floating leaves should absorb the CO2 in the air. I had a similar case to you, I had a contaminate come in with some of my plants; it was just a small water lettuce and now I have a bunch.