Krylon Fusion is safe to use in an aquarium, but as mentioned before, may melt the styrofoam. One possible way to use it directly on the styrofoam without melting it might be to use extremely light coats of paint, just dusting the surface so that there is not much solvent to eat large holes in the foam. Just be sure to read the Fusion label for drying time between coats, as I remember it needs a week's wait if new coats are not applied within a certain timeframe. (This would also indicate a week's time to fully cure.) It does dry fairly quickly, though, so building up repeated light coats shouldn't be difficult, especially considering that any plastic-disolving solvent will disapate quickly. If you have a spare piece of styrofoam, it might be worthwhile to try the light, dusting coats until you get a good layer of paint to see if that method will work for you. Once you get a decent coat covering the styrofoam, you shouldn't have a problem with it melting, since the paint will then be bonding to itself.
Looks like a cool setup; let us know how your project proceeds?!