Hiya,
Have just used slate myself in my new aquarium which is currently cycling. Its great to use to make caves etc. Will post some pictures at a later date.
Basically my partner just used a hammer and chisel to break the bigger pieces down, sand of any sharp edges, and started to make a cave. we had some small pieces and some larger pieces, so just arranged them, and checked it was sturdy and safe (it wasnt going anywhere to damn heavy) and when i put the lights on and added some plants it looks fantastic....
Specially with some bubbles going through from a bubble disk ;-)
Just cant wait for fish now................ grrrrrrrrrr..LOL