Here is some of the things I have made out of styrofoam and coated with Hot Wire Foam Factory’s Exterior Foam Coat (unpainted and not dyed). All of these have about 3 coats on them that I even applied more watery than directed on the package. At only 3 coats, it is really strong! I made one that didn’t come out how I wanted and am trying to break it to get the magnets in it out and I so far have not broken it open. I am pretty sure I am going to have to use a hammer. Better than any grout I have use and it doesn’t require you to mix in anything to make it stick better or strengthen it to this point. You also can use paint to dye the foam coat if desired (it also slows the drying process). As it is drying you can texture it and make patterns in it making it really awesome for making these look like real stone / weathered bone / wood. All of these (one not shown, thats the one im trying to break) I have used a 3 lb container of foam coat for, and I still have about 1/3 of it left. Just remember you still need to seal it after painting or after applying before putting in the tank. You can find this on hotwirefoamfactory.com or amazon, the site that makes this stuff accepts paypal.
HIGHLY recommended for anyone making any foam decor for really anything
Since I can not link things from most sites at this moment heres what it looks like for reference: