What Is KH (Alkalinity)?
KH, or carbonate hardness, measures the concentration of carbonate and bicarbonate ions in aquarium water — this is what most hobbyists mean by “alkalinity.” It does two jobs at once: it buffers the water against sudden pH swings, and in a reef tank it’s the raw material corals pull from the water to build their calcium carbonate skeletons.
Why KH Matters
KH is measured in degrees (dKH) or parts per million (ppm), where 1 dKH ≈ 17.9 ppm. Freshwater community tanks generally tolerate a wide range, but reef tanks need it held stable — most reef keepers target 8–12 dKH and, more importantly, keep it consistent day to day. Corals consume carbonate continuously as they grow, so KH drifts downward over time in an established reef tank and needs regular dosing or water changes to correct.
A KH crash or a large swing (more than about 1–2 dKH in a day) is a common cause of coral stress, tissue recession, and pH instability — testing KH regularly is one of the highest-value habits in reef keeping, alongside tracking calcium and magnesium.
Shop KH & Alkalinity Test Kits
The Marine Aquarium Test Kits category covers dedicated KH/alkalinity testing, including the Salifert Marine Reef Test Kit – KH + Alkalinity. To correct a low reading, see Marine Aquarium Water Conditioner for buffer and dosing products.
