I used to have Bio Balls in my filters for aquariums. Basically just a large marble to golf ball sized plastic ball that has a lot of surface area to grow beneficial bacteria for increased biological filtration. I would think that if a person were to lay them side by side and put layers of say window screen or other porous material between each layer one could eliminate a lot of weight and increase biological filtration immensely. They could even be used without layers depending on the setup. If I ever set up my 55 for frogs I plan to have them in the waterfall reservoir for biological filtration.