Is styrofoam safe to use?

A large block of styrofoam is very buoyant.

Pumice is popular for the purpose, it's light weight. But you said you didn't like using rock, etc.

How about drinking water bottles (with cap on) ?

 

Rocks aren't a big deal if you just throw down some plastic egg crate. I've found the problem with retainers and aquasoil is that they become exposed over time if you push the slope too steep. You should see the rock work that they do for coral or African's with nothing but this stuff down underneath. Most planted tanks don't even see 1/2 that much rock.

Aquasoil drains well, so it doesn't tend to be prone to anaerobic/hypoxic conditions as easily. I've seen the stuff stacked a good 6-8 inches thick over at Aquaforest Aquarium in SF for a glosso-covered iwugami tank. It didn't appear to have any strange discolorations through any of the substrate, but I didn't see any fauna in the tank either. I've pushed it 4-5 inches no problem myself in the past.

Anyhow, if you find a filler of your choice you may want to find an agreeable compound to use for adhering some aquasoil over the easily exposed areas so that the terracing material isn't exposed to easily by substrate settling.

 

What if (yes this includes rocks, but not ginormous rocks) you hollowed out a "pocket" in the top of the syrofoam and filled it with gravel, level to the top of the styrofoam and hollowed out another "pocket" in the bottom to reduce some of the bouyancy? You're kind of stuck having to use my idea, as I only have one good idea a year and this was it.

On a side note, how did the search for a store up there turn out?