Ok, so I am trying to make a paludarium. I started constructing my background out of extruded polystyrene foam and 100% silicone. Everything was going great. (It even looks good!)

I then sprayed the back (just the back, thankfully [I hope]) with black Krylon Fusion spray paint as I've read about how it is safe for aquariums and how others have had success with it on blue foam. I, however, don't seem to be. The paint made the foam on the back "melt." There are little holes where the paint ate through a bit all over it.

Have I just made my structure unsafe for animals and completely ruined all my work?! Is there a way to fix it?

Also, how can I continue (or, God help me, start over) without the same thing happening again?

I think those spray paints contain certain chemicals that can melt certain plastics.. like the foam you have. You may try to spread some of that silicone over the holes in the parts that melted. If you want to paint it again, maybe try to look for some kind of acrylic brush on paint? i'm not sure though.