Honest opinion

[psuedo]

the emerald crab will move when you move the rock. I agree with you for th efact that you should have a cave, and that you need the powerhead to be a little lower. One way to do all this is if there is one corner that is not visible to the viewers, then you could build it up a little with eggcrate and zapstraps. It will make the tank look more structured rather than it all laying in a pile. The fish and inverts will appreciate it, and it will be easier to place corals in the long run.

I like to hide my powerheads behind a rock so as the rock deflects the current.

Everything said here is my opinion and that is how I would do it, but The tank looks good now.

Love the barebottom tank. I am just redoing my tank right now with a barebottom. I know it is a pain in the ass to get the rockwork the way you like it and I have found that eggcrate helps a lot, as I can also drill pieces and zapstrap them to the eggcrate.

Good luck