Let it sit in the bucket with fresh water and an airstone for a couple of weeks to really make sure you leach anything out of it that may be in there ( refresh the water 2-3 times each week ). Also, buy yourself a cheap NEW scrubbing brush, and scrub the bugger all over with plain hot water ( careful not to burn yourself! )
It really depends where you got the slate I think. If you bought it from a fish store or something like that, then it may be safe after a few boilings and scrubbings, but if from a quarry/outside/builders merchants, it would be on the safe side to do the bucket and airstone as well as the boiling and scrubbing to make double sure.
You should do this with all rocks collected as you don't know what they have been exposed to or sitting in.