Certain lava rock can be VERY tricky in a saltwater tank and isn't reef safe...those small pits often contain large amounts of metals which could dissolve/break down in the tank and kill everything. It's used in FW
& brackish tanks more then SW
due to that risk.
Base rock cost's almost the same and is a much safer alternative. If seeded with lr
the base will become live within a year or so.
Marco's base rock
is reasonably priced.