The short answer
A buffer is what keeps your pH from moving. In a freshwater aquarium, the buffer is your KH (carbonate hardness) — the carbonates and bicarbonates dissolved in the water. They act like a chemical shock absorber, neutralising the acids that constantly build up from waste and CO2 so pH stays steady. Plenty of buffer means a rock-solid pH; little or no buffer means pH drifts and can crash. Maintaining a healthy buffer is the secret to a stable, low-drama tank.
How buffering works
Every day your tank produces acid — from the nitrogen cycle, fish respiration, driftwood and CO2. Without a buffer, each bit of acid pulls pH down. KH soaks those acids up, holding pH in place until the buffer is used up. That’s why two tanks can start at the same pH but behave completely differently: the one with higher KH stays put, while the low-KH one slides. See KH and GH explained.
When the buffer runs out
If KH falls to near zero, there’s nothing left to absorb acid and pH can plummet overnight — a pH crash. This is the single most common cause of sudden, unexplained pH problems. Test KH regularly with a liquid test kit so you catch a thinning buffer before it fails. See why pH drops and how to fix a crash.
Keeping a healthy buffer
Regular water changes refresh KH from your tap water. If your source water is soft, add crushed coral to the filter or substrate for a slow, self-limiting boost, or dose a carbonate buffer. See making water harder. More in the water testing hub.