Just a note on you trying to make it sink. forget about that. You have to silicone it in. Try filling the tank with water and hold that all down. You will be in the battle of your life.
I made a center section of my tanks background from spray foam in a can. I wanted to hide the heater and my sump return line.
I sprayed this on a piece of plexi. I put inflated ballons in it to make caves. When the foam sets, I could pop the ballons, instand cave. Threw some sand on it to make it look somewhat natural while to foam was setting up. I had to silicone this to the back wall. it is very powerful when it comes to floating.
With all the foam you are using, which is like 80% more than I did. Your tank will want to float away. Okay, only background. But it will try and break apart if you dont have all of it attached.
I trashed mine as it broke down over the years.
BTW, what you have made is far more impressive than what mine looked like. Mine kind of looked like 1/2 a volcano.
Just giving you a heads up.
Is your avatar a supercharged 5.0?