One of my biggest concerns with the background is the amount of foam that I used and the risk that the entire thing will float up to the top after I fill the tank with water. To mitigate those risks, I filled the structure with river rocks and used spray foam to fill in the gaps. The foam will help bind all of the foam boards together along with the glue and plastic popsicle sticks that I used for the internal structure. The river rocks were to offset the buoyancy that the extra foam might cause. Also, the structure is big enough that it fits tightly in place under the frame of the tank. The junctions of the three foam sections also are shaped in such a way as you can't just pull them straight out. I plan to use a ton of silicone to affix this background to the tank glass underneath, on the sides and on the back of the background structure. The Drylok application made the fit even tighter. It fits so tightly that I can't pull it straight out to the front of the tank. I think that it might not need the silicone to stay in place, but I'm not going to take a chance on not using the silicone.
Last night, I fit the structure into the tank. The tight fit discussed above created another problem. Not only will it be challenging to apply the silicone neatly, but I had to really push to get it into place. The tight squeeze caused about a quarter sized spot of Drylok to chip away at the junction of the center and right sections. I'm going to pull it out and repaint that spot, but if it happens in during the final installation, then I'll just have to do an in-tank touch up. I'd rather not do that, but....I gotta get it done.
Here's what it looks like from the left side, you can't see the spot that chipped away. The spots that you see are just a reflection off of the glass.
You can see where the paint chipped away near the top of the junction of the middle and right sections, leaving the pink foam exposed. It should be easy to fix, so I'm not that worried about it.