GH (general hardness) measures calcium and magnesium — minerals fish and plants use directly. KH (carbonate hardness) measures carbonate/bicarbonate ions that buffer the water against pH swings. Most community fish do well at GH 4-8 dGH and KH 3-6 dKH, but the right target depends on the species you keep — low KH in particular is the most common hidden cause of sudden pH crashes.
New aquarium keepers often assume GH and KH measure the same thing, or that testing pH alone tells the full story. They don't. GH describes the concentration of dissolved calcium and magnesium — minerals that plants and fish actively use for growth, shell and bone development, and cellular function. KH describes carbonate and bicarbonate ion concentration, which has an entirely different role: buffering the water against pH change. A tank can run high GH with low KH, or the reverse — one number tells you nothing about the other.
Of the two, KH is the more operationally important number for day-to-day stability. Biological processes in an established tank — the nitrogen cycle, fish respiration, CO2 injection in planted setups — constantly produce acids that push pH downward. KH acts as a chemical buffer, neutralizing that acid production and holding pH steady. When KH drops too low (below roughly 3 dKH), the water loses its buffering capacity, and pH can crash suddenly — often overnight, with no obvious warning sign beyond the KH test itself.
There's no single universal target — the right range depends entirely on what you're keeping:
Tap water hardness varies enormously by region, so testing your source water before stocking is worth the five minutes it takes — a keeper in a naturally hard-water region will find hard-water species dramatically easier to maintain long-term than fighting the tap toward soft-water conditions indefinitely.
Commercial GH/KH buffer products, dosed to the water column, offer the most controlled adjustment — useful when remineralizing RO or distilled water, which carries almost no natural hardness. Crushed coral or aragonite substrate additives raise both GH and KH gradually through slow mineral dissolution, a gentler but less precise method suited to hard-water setups. Whatever method you use, adjust gradually — sudden hardness swings stress fish as much as sudden pH swings do.
Lowering hardness is the opposite challenge and comes up just as often, usually for keepers with naturally hard tap water trying to house soft-water species. RO or distilled water, blended with tap water to reach a target hardness rather than used at full strength, is the most predictable method — full-strength RO water carries essentially zero minerals and needs remineralizing before use, or it can cause its own health problems. Driftwood and peat also soften water gradually, though the effect is harder to measure and control precisely than a blending approach, and both tend to drop pH alongside hardness, which is worth planning for rather than discovering after the fact.
Liquid test kits report GH and KH in degrees (dGH/dKH) or sometimes in parts per million — check which unit your kit uses before comparing to any target range, since a mix-up between the two reads as a number roughly 17.9 times off. Titration-style kits (counting drops in a small water sample until a color change occurs) are more accurate than dip strips for both GH and KH, and strips in particular tend to understate KH at the low end, right where the pH-crash risk is highest.
Our complete freshwater guide covers the full parameter picture — pH, hardness, nitrogen cycle and filtration together.