Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Any rock that is limestone, calcite, dolomite or related will leach mostly calcium and magnesium carbonates. Many rocks are in this group with lots of names. Some are harder than others, so leach more slowly. Others are so soft you can practically see them melting in the tank, especially if the water is kept on the acidic side.
Most harder rocks like quartz, granite, basalt and lava tend not to release minerals. However, sometimes these rocks can have veins of softer minerals in them. Then the veins might dissolve over time, but the main rock does not.
The easiest way to tell if a particular rock will leach minerals under your conditions will be to test.
Set up a jar of water with tap water or whatever water you will be using in the tank.
Add whatever additives you will be using in the tank.
Test this blend to get a base level. What is the water like before you add anything.
Then add rock samples. The finer they are broken up, the more surface area, the faster you will see results. If you have to break up the rocks wear eye protection! Rocks can chip and fly when you hit them with a hammer.
Test for all the aquarium tests that are available such as:
TDS: Total Dissolved Solids will let you know SOMETHING is in the water, but won't tell you what it is.
GH: Tests Calcium and Magnesium.
KH: Tests carbonate and bicarbonate.
pH: is not actually testing minerals, but might change to let you know something is going on.
Other tests based on what kind of rock you think it is: Iron, Phosphorus, Calcium, and whatever other tests you see.
There are some good web sites that can help you find out what the local rocks are in the hills near you, and can help you ID rocks from elsewhere. You could try posting pictures here, too. Explain where you found the rock.