sorry but it will not work. I would stop drilling the the rock now... Featherlite rock is also known as pumice rock almost the same thing as a pumice stone you use for cosmetics. They naturally are filled with thousands of tiny pockets of gas. Honestly I would only suggest using a glue to attach it to the bottom of the aquarium (clearly the aquarium has to be empty to do it.) or if your willing to use other types of none lava rocks, you could use aquarium safe adhesive to attach it to another rock to force it to sink.