I have been wondering the same thing. Back in the 70's I had a 300 gallon saltwater tank. Back then things were a lot more primitive, but I used a lot of lava rock. It was believed at that time that lava rock was the only rock safe to use because the extreme heat removed all of the metal from the rock. I am hoping to get back into this hobby, and i really like the look of the lava rock. It didn't seem to create any problems back then, but a lot was "hit and miss", and we didn't keep any corals.