KH/Alk

KH, or carbonate hardness, measures the concentration of carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) ions in water. These ions act as bases and play a major role in stabilizing pH. Because bicarbonate can both accept and donate hydrogen ions depending on the water’s pH, it acts as a buffer—resisting sudden swings in acidity or alkalinity. That’s why tap water, which often contains both carbonate and bicarbonate, tends to sit above pH 7. These ions are completely harmless to fish, but they do contribute to the “extra stuff” dissolved in your water.

Now, alkalinity is a broader term. It refers to the overall buffering capacity of the water—how well it can resist changes in pH. Carbonates and bicarbonates are the main contributors to alkalinity in most aquariums, which is why KH is often used as a rough estimate. But they’re not the only players. Other compounds, like phosphates and even nitrite, can also contribute to alkalinity without affecting KH directly. So while KH and alkalinity often go hand-in-hand, they’re not the same thing.