GH & KH: The Backbone of Water Chemistry

When aquarists talk about "hard" or "soft" water, they are usually referring to two distinct parameters: GH and KH. Understanding the difference is vital for plant growth and pH stability.

GH (General Hardness)

What is it? GH measures the concentration of Magnesium (Mg²⁺) and Calcium (Ca²⁺) ions in the water.

Why it matters:

• For Fish: Fish need these minerals for bone development and osmoregulation. Livebearers (Guppies) need high GH; Discus prefer low GH.
• For Plants: Calcium and Magnesium are essential nutrients. Low GH causes twisted leaves and stunted growth.
• For Shrimp: Vital for proper molting. Without enough GH, shrimp fail to molt and die.

KH (Carbonate Hardness)

What is it? KH (also called Alkalinity) measures the concentration of Carbonate (CO₃²⁻) and Bicarbonate (HCO₃⁻) ions.

Why it matters: KH acts as a Buffer for your pH. Think of KH as a sponge that absorbs acids.

• High KH (4+ dKH): Your pH will be very stable and resistant to change.
• Low KH (0-2 dKH): Your pH is vulnerable. Acids from waste or CO2 can cause the pH to drop rapidly (a pH crash).

Measuring Hardness

We usually measure in Degrees (dGH / dKH) or PPM (Parts Per Million).

• 1 dGH ≈ 17.9 ppm.

| Range (dGH) | Classification | Typical Inhabitants | | :--- | :--- | :--- | | 0 - 3 | Very Soft | Discus, Crystal Red Shrimp, specialized plants | | 3 - 6 | Soft | Tetras, Rasboras, most South American fish | | 6 - 12 | Moderately Hard | Most community fish, Rainbowfish | | 12 - 18 | Hard | Livebearers (Guppies, Mollies), Goldfish | | 18+ | Liquid Rock | African Cichlids |

"Old Tank Syndrome"

KH is consumed over time by the nitrification process (beneficial bacteria use carbonates). If you do not do water changes, your KH will slowly drop to 0. once it hits 0, the pH is no longer buffered and will crash dramatically, killing your fish. This is called Old Tank Syndrome. Regular water changes replenish KH and prevent this!