Some questions (since I've built a foam/rock wall and I'm assuming you're also using foam):
- How big is your tank?
- Is there anything living in it yet?
- Can you drain at least some of the water out?
Constructing the wall itself doesn't take long. Mine's not very big (about 22"x30") and it only took a few days, but most of that was curing time. The foam had to cure and then the 2-part epoxy & sand layer that was used to cover the foam. It is siliconed to the back so that was more wait time but I gotta say, it was pretty much stuck immediately even before the silicone cured.
One of the reasons I did the rock wall instead of piling rocks up was because of quakes. If you don't seal it to the back there is a chance it could fall forward and smash the front of your tank. In a normal quake probably not but I remember Northridge (thrust fault) where people were thrown from their beds. My 40g freshwater acrylic tank stayed up but lost most of its water. My boss's 100g acrylic freshwater tank moved 6" on its stand. Another friend lost a glass 220g because the LR fell and took out the front. So I guess my advice would be to not put something that big and heavy in without securing it.
Let me know if you have more questions, I love my wall but there are definitely pros and cons with it.