I dont know if this could have anything to do with it, but I would remove the slate. I know that its bad to put coral in freshwater tanks or anything for that matter that isnt labeled aquarium safe. The slate might be poisoning the water, but thats my guess. As for the LFS testing your water, as I said in an earlier reply, get your own test kit. If you do get them to test your water, tell them you want to know the readings in ppm, not that they are ok or fine. Anything other than 0 for ammonia and nitrite is a problem and not ok.