I was talking to someone about slate and river rock in my area. He said to keep in mind that alot of river rocks will leach hard minerals into the water. Even though it may have no immediate effects, it could cause a build-up of unwanted minerals in the water over the long term. The guy I was talking to was telling me how he was building some 'caves' for his Tigrinus catfish, out of a slate he had boughten at a gardening store. He said after about a week the Tigrinus started breathing very hard, and died a few days later. He couldn't figure it out, until he took his water in to get it thoroughly tested. It had high amounts of trace copper in it, which would cause the irritation of the gills on the Tig, leading to it's death.
Just thought I would throw that in! Be careful what you put in your tanks!