Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Sounds like any metals are bound up really well between the volcanic activity and other things that have gone into creating that interesting rock.
I have some rock in one tank that has actual iron rusting in place. That is the sort of metal that you do not want in an aquarium. The actual ore that changes in the water, and is released into the water. Copper is especially bad. Iron is a plant nutrient, but I do not know if they can take in this form. It is only a few coin sized spots and the tank is 125 gallons.
Put a few of those rocks in a bucket of tap water. Test GH, KH, pH, and TDS when you start, and every few days for a week. If there are minor changes, then it is probably safe. If there are major changes in the GH, KH and pH (rising values) then the rock is probably fine for a hard water tank, but not soft water tank.
If the only thing that changes is the TDS, then I would want to know what is coming out of the rock and into the water.
GH is calcium and magnesium.
KH is carbonate, a buffer that stabilizes the pH.
These 2 are generally found in limestone and related materials.