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I don't think the thin ones are structurally sound enough for a sheet supported only at the ends to hold frags in a large frag container, like a 150 Rubbermaid stock tank.
Gotcha. We actually did use the thinner stuff. What we ended up doing was putting feet/supports every so often to support the egg crate. In the 125 we strapped a piece of 1" pvc down the length of it to make it a bit more rigid.
But the thicker stuff would certainly make things easier.
Is it OK to put eggcrate under the rockwork to prevent rocks from scratching/breaking the bottom?
After the rockwork in place I would cover eggcrate with live sand.
Will it become a detrius trap or a haven for anaerobic microbs?
This thread was a year old, but still a valid question on the topic.
Yes, you can put (and I'd recommend it) egg crate under your rockwork. Covering it with sand might seem like it would "lock-in" the detritis, but bristle worms will make their way down there and take care of it as long as you have at least an inch of sand cover.