I worked in a rock yard for two years and got this question all the time. Slate is a good choice, but does have very sharp edges which a fish can cut themsleves on. When i would set up tanks (chiclid)for people i would try to use a limestone, (we called it Monterey Creme, or Santa Margirita, its basically whiteish in color, the names may be different form yard to yard) which will help riase the pH. You want to say away from rocks that have iron in them as well as sandstone. The rocks with iron are reddish, or look like they have rust colors in them, you dont want the iron leaching into the water. Sandstone will lower the pH. Clean as recomended, most rocks will work i found the limestone will work the best for cichlids.