The stuff that comes in a can to seal windows and doors that is a type of expanding foam be safe for use in an aquarium after it is cured? Ive read in diy posts that it can be used but never heard wether it worked on the background that was being made. any and all help would be appreciated thank you. also could you dig the tunnels of caves out before applying cement to make it easier to coat the inside of the tunnel before siliconing the styrofoam together.