Eggcrate Under Sand ?
 

I've used eggcrate under sand where live rock was located (to relieve and stress-points) in the past and had no issues but for a new tank I'm building, I'm contemplating putting a sheet of eggcrate covering the entire bottom of the tank to make changing aquascapes easier in the future.

I'm looking at about 1 - 1.5" of sand above the eggcrate.

Anyone have an opinion on why this may or may not be a good idea. The obvious concern is that it may impede sand dwellers, sand sifters and the like. Anyone done this and regretted it ?

I have 100% of my tanks bottom covered for a year and a half now. I've had a YWG, twin spot blenny, sand sifting star fish, nassaruis snails, etc and no one has seemed to mind.

Yup, I did it, dumbest thing I ever did. And now that I want it out, I can't remove it. I read it and thought "hey, I'll do that" for no real reason, and it was just dumb. The front part is exposed, it impedes sand dwellers and it doesn't actually do anything useful. It is not needed to relieve stress from rocks.

How deep a sand bed do you have over the eggcrate ?

We had a good discussion about this early in the year. I ended up going with live rock directly on the bottom, with a shallow layer of sand all around.
http://www.canreef.com/vbulletin/showthread.php?t=81984

Depending on water flow you could potentially have egg crate show through as the sand is moved around by currents and critters I have egg crate on the entire bottom of my tank and I would rather have that extra layer of protection and maybe some egg crate showing than a cracked or broken tank. As to critter impediment I don't believe there will be an issue there.

So do you put the rocks on the bare glass and then add sand or put sand and lay the rock on the sand ?

I figure the latter would remove the need for the eggcrate in the first place but I'm concerned that sand dwellers could undermine the rock structures and cause collapses...

Mine's about 1.5"

Unless you're tossing rocks around in the tank, it's not going to break. Even if you tossed them, they'd have to hit hard.

Put it on the glass. This prevents anything from digging under it and toppling your rock scape. I would actually put a bit of sand, add the rock, twist it into the sand a bit so that it's touching glass, then add the rest of the sand.

The discussion linked above, if it's the one I'm thinking of, will give lots of input. But rock won't break glass just by sitting on it.