I used egg crate under my rock and don't find it has caused any problems, but second the opinion to keep it away from the glass - especially in front, or you will see it if the sand shifts. I have a 2 inch sandbed and have a jawfish who seems happy. I don't see the egg crate so it doesn't cause me any concern & when I switched tanks over a few years back when the tank was about 5 years old the sand was still great and there were no pockets of yucky stuff. I definately wouldn't cure rock with sand or eggcrate though - when I did that I did it barebottom and did a ton of water changes.
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65 g mixed fish with various lps, 50 lbs live rock, remora skimmer, 2 A150W sky blue Kessils
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