Today i started the upgrade from my 20 gal planted to 55 gal planted with a custom background. I used Plexiglas, sand paper, pebble, Styrofoam, and silicone caulk. To seal the Styrofoam I first covered the whole shape in caulk, and then let it dry. Once dried I reapplied caulk, and added the pebbles big one first then smaller ones. The pebbles were separated by size I used a kitchen colander and used the small ones as filler for the small holes. Yes I put the rocks in the oven to dry them out as I have little to no patience.