Hey Nate. Super glue gel should work fine. Dry the back of the piece you want to add as best you can. Put a pretty good pile of superglue and put the frag in the tank. Normally it works best to put the glued part in a small recession or hole where you want it. When you have it in the tank, wiggle it around a little bit and see if it stays. I find if it doesn't stay at first, pull up on the frag a little bit to expose the glue, then press it back down with a slight wiggle again. It takes a little time to figure it out at first, but you'll start to get the hang of it.
tom