I have a pretty big hunk in my tank and it's still not able to stay under by itself. When I first got it, I ran hot water over it, kept it submersed in a garbage pail (specifically meant for aquarium stuff) for a week or so....nothing. This is already screwed onto a piece of slate, so the slate isn't helping...... went to the LFS and bought 3 big rocks and burried the slate under the gravel as well as placing the rocks on the corners (3 of them) of the slate. To this day, if I moved those rocks, that thing would be floating, although it wouldn't torpedo up to the surface like it used to...