I would actually recommend drilling the rock and then attaching it to some eggcrate that sits in the back if you're looking to make a 3d backdrop. I would be nervous about putting something other than straight silicone into the tank as far as adhesive agents go. There are a couple of other builds on here that involve sculpting backgrounds with sprayfoam coated with concrete as well.
The only other thing I can think of though is epoxy. If it works you're in for a permanent bond, but I don't know what the solvent in epoxy would do to an acrylic tank wall. There's a chance it could melt into and weaken the tank wall, which wouldn't be good. I would test it out on a spare piece of acrylic first.