The short answer
GH (general hardness) measures the dissolved minerals in your water — mainly calcium and magnesium. KH (carbonate hardness) measures carbonates and bicarbonates, which act as a buffer that keeps your pH stable. In short: GH is about the minerals fish and shrimp need, and KH is about how steady your pH stays. They are two different tests and should be measured separately.
GH — the minerals that matter to livestock
GH tells you how “mineral-rich” your water is. It matters most for shrimp, snails and livebearers (guppies, mollies, platies), which need calcium and magnesium for shells, moulting and general health. Soft-water fish like many tetras and bettas are happier at the low end. GH doesn’t directly affect pH — it’s about what’s dissolved for your animals to use, not about acidity.
KH — the buffer that steadies pH
KH is the more important number for tank stability. Carbonates neutralise the acids that naturally build up from fish waste and CO2, holding your pH steady between water changes. When KH is very low, there’s nothing to absorb those acids, so pH can drift down or crash overnight. If you struggle with a wandering pH, KH is almost always the reason.
Testing and adjusting
Measure both with a liquid test kit — dip strips are fine for a rough check but drift over time. To raise GH and KH, add crushed coral or a mineral additive; to lower them, cut tap water with RO or rainwater. See how to soften water and how to make it harder, and understand the link between KH and pH in why pH drops. More background lives in our water testing hub.