I know this is an old thread, but I'm in the middle of making another foam background for someone and thought I'd throw this out for you (since I saw it).
Instead of using regular Styrofoam from the craft isle, walk a few isles over to the floral isle. Grab some floral foam. It's very smooth and almost carves itself. You'll be amazed how easy it is. I use a flat head screwdriver to carve with, if that tells you anything. It scrapes easily away with minimal effort. Regular Styrofoam .. you gotta put some muscle to it.
It makes a fine dust though, so you need to wear goggles and one of those white masks. You can get a pack for cheap on the home & garden isle. You don't wanna breathe this stuff, or get it in your eyes. I've gotten it in my eyes .. it's not nice.
You also don't have to use quickrete to coat it and make it look like rocks either. Simply paint the piece you've carved some basic color I usually do it a light brown). Let it dry. Then you can do it one of 2 ways.
Materials needed:
Acrylic Craft Paint (you decide the colors based on what color you want your rocks)
Colored Sand (again, up to you on the color. Can be bought at most Hobby Lobby's for $1.99 for a 1 1/2 lb jar).
Clear Polyurethane, waterproof variety (can get at most Lowe's)
Way 1. After coat 1 dries paint rocks the color you want. Sprinkle with light colored sand as you go. Let this dry completely. Apply 2 coats of Polyurethane, let dry completely between coats. Then let it cure for 2 weeks. Letting it dry outside (where there is heat, but no moisture) works best. (this makes sure there are no chemicals left. They will evaporate).
Way 2. Paint your rocks whatever color you want. Apply a coat of Polyurethane, let it dry completely. Apply your second coat and sprinkle with whatever color(s) of sand you want your rocks to be. Let the piece)s) cure for 2 weeks.
Way 1 will give you smoother rocks, Way 2 will give you textured rocks. You can also add way more detail with Way 1 than Way 2. I prefer Way 1 myself.
Here are some examples from last night (they do not have Polyurethane on them yet though. As they aren't completely dry .. plus my Polyurethane hasn't come in yet lol.)
They will look rather shiny after you put the poly on .. but once it's in the water .. you can barely tell. Once they've been in the tank about a month .. they look completely real.
Here are some that were done in Way 2. (it's not all of the same piece .. these are just to show the steps)
foam block
carved
painted
1 coat of poly
sanded (I did several layers , this was layer 1)
sanded (layer 2)
You will have to use either epoxy or silicone to attach it to your tank if you aren't using the quickrete though. I like this way better because it doesn't add so much weight sitting on the floor of your tank. I also like it because concrete will sometimes raise your PH, which is great if you have cichilds, but not so great for other fish.
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