Okay, I have read a lot of different way s that people have made backgrounds from Styrofoam and concrete or Dry-lok and am going to try it on my already established 55 gallon tank. I have read that it is possible to place a background that is tight fitting in an already established tank and use the frame to attempt to keep it in place, as well as rocks and substrate at the bottom. However in addition to that I have purchased a set of 8 plastic coated aquarium safe magnets that I was going to use on the background as well. I was going to recess them on the back and attach them with silicone. Having an established tank, I cannot use silicone, and I really would like the option of removal in the future. I know there is pond silicone that can be applied in the water, but again I am not looking for a permanent fixture. What I was thinking of doing was using 1” extruded polystyrene foam, cutting it to fit and then attaching rocks to it that I will cut out of the same polystyrene to make low profile ledges and small depressions/pots to hold some Anubias and possibly stem plants on the top to keep them “floating” in place at the top of my tank. I would glue all the pieces onto the foam with silicone, and let it cure for a few days or week and then cover it with a couple coats of tinted Dry-lok. I was thinking of outlining the whole frame with a bead or thin layer of silicone just so that it would “grip” the sides of the tank as well as the adjacent piece better and keep itself in place better. I also wanted to “hide” some of my intakes as I have 2 Filstar canister filters for the tank, and make a mes screen cover out of aluminum to keep some of the debris from clogging my intake tubes. I am unsure if I should make it in two 24" or three 16" pieces, also since I am not attaching it with silicon, can I paint the back of it as well with the Dry-lok? I think this will help with the longevity of the background [FONT=Lucida Grande, Trebuchet MS, Verdana, Helvetica, Arial, sans-serif]I also wanted to attach it to a 4” or 5” “base” that would just be another 1” piece of foam, painted with Dry-lok and silicone to the bottom of the background…a “foot” that I can cover with my substrate as added protection from it floating or shifting. Do you think this would help with the inherent buoyancy of the foam, or it is overkill?[/FONT] Thank you for your help and feedback!