The bottom of mine has a fabric planter bag of kitty litter - a planted pot that I accidentally knocked off of a shelf! Now it is just down there... I have a larger pot with a water lily too. That is really all that I put on the bottom. However, some anacharis has made a little forest - hornwort too. I started with them tied to smallish rocks and had them on the medium shelves. Two plants have also become borderline invasive, sending crazy long stems meandering all across the bottom and sending up stalks with leaves at the surface. Yellow heart (aka frog bit?) and I believe it's called water willow. They were pass along plants and I really like them! Luckily, they are easy to pull out. Periodically, I lower the water level a bit and just get in and pull.