I saw the pictures you had forwarded to Joseph, and still not sure from those pictures as to the exact identity of this rock, other then it having quartz veins. Indeed sandstone is usually soft and often brittle, but its sometimes more durable. It doesn't look like any calcite is in there, so you would probably be safe on that end from hardening the water. Its also less likely to be metallic sulfide, which can leach metals, but there isn't a way to tell forsure by just your pictures. While it will most likely not cause an issue, I would suggest you play it safe when putting it in a fishtank.
