Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
You can get rock in places that sell it in bulk. Most will sell it by the pound, just pick out the ones you like. Get more than you think you need, some will not work when you get to arranging them.
The general term for relatively flat rock is flagstone.
This does not tell you what minerals are in it, just the shape.
Take some baggies with you and a sharpie marker.
Take some samples of whatever rock you like. Little bits break off and fall from the pallets all the time. If the rock is small, put a few pieces in a baggie and label it. Larger pieces can be labeled right on the rock.
Then take the rock home and clean them (scrub with a stiff brush), then put each type in a different jar of water. Test the water when you start, then every few days for a week.
If the GH, KH, TDS and pH change only a tiny bit, or do not change, then the rock is safe for use in soft water tanks.
If these parameters change significantly then these rocks might be OK for a hard water tank, but not a soft water tank.