Putting it right under the rock isn't going to protect your glass if you were to drop a piece somewhere else later. So why bother? It's also bound to get exposed and looks ugly. Biggest reason in my mind is wrasses though. Mine dive full speed into the sand and move pretty fast under the sand finding a sweet spot. They're clearly hitting glass and then sliding along it. I can easily see them getting beat up doing this on eggcrate.

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.

So, your recommendation would be just to set it directly on the glass? That was actually my initial plan, and have a thin sand bed around it (maybe 1 inch deep), which would still protect my tank bottom from a falling rock.

BB now and rock direct on the glass but when I had the DSB made little boxes out and eggcrate and zip-ties to support the rock. This was so I wasn't burying the rock under 4+ inches of sand and by having the platforms rather than continuous support still allowed critters to burrow around the LR.

Has anyone actually heard of rock falling and breaking a tank bottom? I wouldn't think it likely, they fall in slow motion in the water... but you never know. I think a piece of 1/8" Plexiglas would be better than egg crate if you're worried about your glass.

I would still raise the back of your rock up on stilts to get flow across the bottom of your rock. Any rock surface buried in sand is wasted. Leave the door open to go bare bottom if you find your nutrient levels aren't where you want them.

I highly doubt it's possible in reefing. You either have manageable sized rocks in a smaller thinner-bottomed tank or you have massive boulders with a 19mm bottom or thicker. I can't see it happening and the risk at the very worst can't be great.

I did drop a rock and blow the bottom of my frontosa tank out back in the day though. They were juvies and the tank was only a 45 gallon. Difference being I was using heavy rundle rock slabs. Our rock is WAY lighter you're right.

So yeah rock right on glass. I'd consider pegs of some sort if they're short to elevate the rock but I'd never do it with eggcrate.

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So what happens if one grain of sand gets between the egg crate or oyster shell and glass?

I'm sorry, but that fringes on ridiculous. The rock is not going to focus it's mass on one grain of sand and break glass. ""IF"" it focused that much mass on a single grain of sand, the sand will crush. Or it will puncture into the bottom of the rock. Or get squeezed out sideways.

I have always laid my rock on bare glass. I've dropped 5 pound pieces of rock in the tank (with water) and had it hit glass. The eggcrate is not dispersing weight to save glass. Just not happening. My holding tank has 3/8" glass on the bottom, I filled it for months with 140 pounds of rock that I flipped and flopped around while rummaging for certain pieces.

Here's an example. I needed to remove a cross brace from a tank. Figured a hammer was the quickest way. It was utterly amazing the force I had to hit it with to actually break it. A rock tumbling 8 inches in water to rest on your glass is not going to break anything but corals.

I have seen once, and only once, where a rock, about 40 pounds, fell 2' in huge tank and actually broke the bottom.

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