Whats a good sealer to apply to foam?

Im working on a little home project for my fish, just trying to make a nice 3d background for them out of styrofoam but i heard after time the foam will fall apart. my question is, is there a good sealer i could use on the foam that wont hurt the fish? i could use Silicon but i don't know how i would smooth it out and get it in to those nooks and cranny's, i tried it with cement but that was a mistake. any help would be awesome.