protecting underground spaces

[seaburst]

I've been researching scaping Cichlid tanks, and I found out it's best to place rocks first and surround them with sand so the glass doesn't crack if a fish decides to dig. So, what's a good base layer to use? Something that won't rot, I assume?

[dorvex]

I've seen people use eggcrate for the base layer, but there's also a thin rubber sheet you can get that goes between the tank and the rocks. Personally, I just place the rocks straight onto the tank base.