Nice, I picked up a featherock boulder a while back, thinking of using it in a paludarium.
It's pumice, so it's really light (most pieces float initially), and it's fairly easy to break up and shape.
All of my projects are currently in hiatus, but I'd still like to try and do something with what I have sometime in the future. Any special issues you noticed working with it? It is indeed sharp and abrasive - it's basically foamed glass. I use gloves when I'm working with it, and wetting it helps to keep the dust down (which you definitely don't want to breath in...)
Did you get smaller pieces, or carve up a bigger piece? And did you just use silicone to attach it? I'm mostly just curious since it's not a method I've used before, but was considering, so talking to someone who actually tried it might prevent some really basic mistakes.
Thanks, and the tank looks nice.