In general, lava rock is not the best choice for a salt water tank. It'll grow bacteria in all the pores, but lava rock has the potential to leach metals into the water, making it less than ideal if you plan to keep any invertebrates in your marine tank. Even months to years of success with it in a fw tank doesn't translate to it being ok for a marine tank. Marine water chemistry is somewhat different than fw water chemistry, and if the fw tank was maintained using tap water, there's always the chance that metals in the tap water have sorbed to the lava rock.