I wouldn't do it.

 

I'm not sure if the solvents in the spray paint will melt the spray foam.

What about coating the still wet spray foam with natural colored sand?

 

I won’t use anything that has petroleum product in it which has toxic solvent and may melt your foam. A safe way I’ve done in my shrimp tank is to coat the surface with superglue and dust it with terra cotta powder. It has a natural larva look and I used an electric grinder to powderize a terra cotta pot.

Why not cover your foam rocks wish sand before the foam dries? Both are aquarium safe and the technique is commonly used in making aquarium backgrounds. You could embed a real rock inside the fake one to ensure it won't float when you're done.

So I've already sprayed some foam with no issue. I had looked up that certain people with reef tanks use both rustoleum and krylon with no issue and I was concerned if that specific kind would be still safe.

The reason I didn't use sand while foam is drying is I shaped the foam once dry with sanding and cutting edges to make a more natural look.

I also know its going to float so I planned on siliconing the pieces in place once finished.

 

I won’t use anything that has petroleum product in it which has toxic solvent and may melt your foam. A safe way I’ve done in my shrimp tank is to coat the surface with superglue and dust it with terra cotta powder. It has a natural larva look and I used an electric grinder to powderize a terra cotta pot.

 

The only paint that I've ever used for in-tank application has been Krylon Fusion.

You can also use clear aquarium sealant on top of the expanded foam rocks and then embed sand/grit if you want to try another method. However, I'm not sure how buoyant the expanded foam is and whether the "rocks" would stay submerged.

 

Why not cover your foam rocks wish sand before the foam dries? Both are aquarium safe and the technique is commonly used in making aquarium backgrounds. You could embed a real rock inside the fake one to ensure it won't float when you're done.