Planted Tank Guru
Join Date: Jul 2011
Location: Christiansburg, Va
When making backgrounds, I coat mine with Drylock. But if the foam wouldn't be visible, I don't see why you couldn't simply coat it in silicone (aquarium silicone or any pure silicone)
And foam always wants to float. You can coat it and silicone it to your tank glass - whether the back for a background, or the bottom for rocks/added mass.