Kh is the Carbonate Hardness of the tank, meaning the amount of alkalinity and buffering capacity. Without adequate Kh, which should be at least 4, not enough buffers are in the water to retain a stable Ph. Gh is General Hardness and refers to the amount of calcium and magnesium ions in the water which tells how hard or soft water is. Again a Gh or 4 is perfect for a planted tank.
The Crushed Coral will not bother the fish BUT you have to go slowly only starting with a small amount and slowly increasing it until you reach a Gh and Kh of 4. I would seriously suggest buying an API Gh and Kh testing kit. It has tests for both Gh and Kh testing in it. If you just go by Ph your not going to know what the other numbers are and they are more important than Ph IMO.
The Crushed Coral will not bother the fish BUT you have to go slowly only starting with a small amount and slowly increasing it until you reach a Gh and Kh of 4. I would seriously suggest buying an API Gh and Kh testing kit. It has tests for both Gh and Kh testing in it. If you just go by Ph your not going to know what the other numbers are and they are more important than Ph IMO.