What can I use as a sealer/paint on my aquarium background?

I recently finished making a 3d background for my aquarium. I used stryofoam and carved rocks and caves into it. Now I need to find a non toxic coating to finish it with. I am a bit implusive and didn't research the end finishing! Is the Quickcrete that some people use really non toxic? I would hate to endanger my fish. I appreciate any input anyone can give me. Thanks.