Most lava in the world is composed of basalt (dark brown to black in color, like Hawaii's lava rocks). Basalt exposed to ordinary seawater at low temperatures (like in your aquarium) undergo alteration that results in the uptake of potassium by the basalt and loss of calcium, silica, amd usually magnesium" (The Geochemistry of Natural Waters, 2nd ed., James I. Drever, Prentice Hall 1982, p.277-280). That is saying that the basalt will release calcium, silica, and magniesium in to the water. However, Drever goes on to say that if the pH rises too high, the basalt may remomve magnesium from the water. At what pH this happens, Drever doesn't say, but since the pH of the ocean is so consistent it's safe to assume something way above a ph of 8.2, but best to keep an eye on it.
Lisa
