As I've said, your design is cool. However, if you were to say you were going to scrap it and start all over, I would have a few suggestions. One of the best reasons to use foam in the first place is to move the rock work up onto the back wall, freeing up valuable real estate on the tank bottom, for say, seagrasses. Your design doesn't take advantage of that. In fact it could be easily done with live rock, which wouldn't have buoyancy issues, and would also be much more beneficial to your system than foam. Your design covers two third of the bottom. One good way to combat buoyancy is to tightly fit your wall between the bottom and the top frame of the tank. Other tips: push out air while it cures, voids like plastic bottle caves fill with water, rather than air, less bulky-less floaty, eggcrate makes a great, structural framework to lock the foam into. Hope this helps!