We have a 150g tank that is visible on three sides so we wanted large pieces of lava rock in the center to act as a background. In CA we had no issue in finding large red lava chunks, in WI it's just not possible to find. But we did find volcanic rock called Featherlight. Unfortunately if there's an air pocket then the 20x18x8 rocks float. My dh drilled 6" deep holes in the bottom of the rocks but they still are floating. Question: In the case we can't get the air out, what kind of weights and ties can we use? Dh is worried about them deteriorating or effecting the water quality. The picture has the two large floating rocks being held down by large red bricks.