GH and KH Explained: Aquarium Water Hardness Basics

Beyond the basic ammonia-nitrite-nitrate-pH readings covered in our water parameters guide, general hardness (GH) and carbonate hardness (KH) play an underrated role in long-term tank stability.

General Hardness (GH)

GH measures dissolved calcium and magnesium, directly affecting fish health, especially for species with strong preferences — soft-water species like many South American cichlids and hard-water species like African Rift Lake cichlids sit at opposite ends of this spectrum.

Carbonate Hardness (KH)

KH measures the water’s buffering capacity, essentially its resistance to pH swings. Low KH water can experience sudden, dangerous pH crashes, particularly in heavily planted or CO2-injected tanks.

Adjusting Hardness

Crushed coral or limestone raises GH and KH naturally over time, while RO/DI water (see our RO/DI guide) provides a blank slate for precisely remineralizing to your target hardness.

Why Stability Matters More Than the Exact Number

Most established fish tolerate a range of hardness values reasonably well, but sudden swings caused by inconsistent top-off water or substrate changes are far more stressful than a stable, slightly “wrong” number.