slate is generally inert/safe for aquarium use. I would have thought the gaps/seems in the rectangular/square tiles would look a bit off, but wkndracer's tank looks pretty good (good job).
I've done the back of a 10-gallon with lava rock (my first attempt at a paludarium). I thought it turned out pretty well, but it used an unholy amount of silicone and time
I haven't used slate myself, other then anchoring driftwood