I have 40g tank that's about 4' long and 18" high. I'm thinking of building a set up where the tank is half filled with water and where I have a moss wall in the back below the water line and, above the water line, I want to create pockets I can plant. This means I need to sculpt the back. I can make a "rock" base from styrofoam and great stuff but I need some help identifying a grout-like material to go over the foam and/or a sealer. There are plenty of threads elsewhere on this where people mention grout and various other materials. But I can't find mention of a specific brand that is known to be safe in an aquarium/palludarium. Obviously, if lime leaches from the grout into the water that could be bad. One workaround would be to use colored grout (same as you would use with tile) and then layer on a sealant. There are epoxy paints that people use in ponds as a sealant but they are black and not suitable for this -- I think I need a clear, flat finish. Does anyone have a specific recommendations for either a safe grout I can get from HD or Lowes or online and/or a reco for a sealant that is invisible and safe for fish/inverts? I appreciate the help...