Mastering Alkalinity (KH)

Carbonate hardness is the buffer that keeps pH stable and corals growing. Learn how to read it and hold the line.

What is Alkalinity?

Alkalinity — also called KH or carbonate hardness — is the concentration of carbonates and bicarbonates in your water, measured in dKH.

Think of it as your tank's buffer: it absorbs the acids produced every day and stops pH from crashing. Corals also draw on carbonates to build their skeletons. Natural seawater sits around 6.4–7 dKH (2.3–2.5 meq/L), but most reef keepers deliberately run higher — 8–12 dKH — to fuel faster coral growth.

Low (0)Reef (8-12)High (20)

Low Buffer (< 7 dKH)

Thin carbonate buffer. pH becomes unstable and prone to crashing as acids build up. Corals struggle to build skeleton.

Note: Risk of pH swings, slow coral growth

Reef Target (8 - 12 dKH)

The stable buffering zone for reef tanks. Provides carbonates for coral calcification and keeps pH steady against daily acid production.

Note: SPS/LPS corals, clams, coralline algae

High Alkalinity (> 12 dKH)

Excess carbonates can burn SPS coral tips, especially when calcium or nutrients are out of balance. Bring it down slowly, never in one jump.

Note: Over-dosing buffer, tip burn risk

Why Alkalinity Matters

pH Stability

A strong carbonate buffer neutralises the acids from respiration and nitrification, preventing the sudden pH crashes that stress livestock.

Coral Calcification

Corals and coralline algae pull carbonates from the water to build skeleton. Stable KH is the single biggest driver of healthy reef growth.

The Balance Triangle

Alkalinity, calcium, and magnesium are linked. Magnesium holds the other two in solution — ignore it and your KH will refuse to stay put.

SPS vs. LPS: Does Target KH Differ by Coral Type?

SPS-Dominant Tanks

Small-polyp stony corals calcify fast and pull carbonates hard. Many SPS keepers run 9–11 dKH and dose on a tight, predictable schedule to keep up with demand.

LPS-Dominant Tanks

Large-polyp stony corals calcify more slowly and tolerate a wider band. 8–9 dKH is a common target — the low end of the reef band, closer to natural seawater, with less day-to-day dosing pressure.

Practical Tips for Managing Reef Alkalinity

Test One or Two Times a Week

Reef demand changes as corals grow. Track KH on a schedule so you catch a downward trend before it becomes a pH crash.

Dose, Don't Shock

Raise alkalinity gradually — no more than ~1 dKH per day. Two-part solutions or kalkwasser let you keep KH topped up without sudden swings.

Balance the Trio

Alkalinity, calcium, and magnesium work together. If KH won't hold, check magnesium first — low magnesium makes calcium and carbonate precipitate out.

KH Stabilises pH

Carbonate hardness is your pH shock absorber. A healthy 8–12 dKH buffer resists the acid produced by respiration and the nitrogen cycle.

The Golden Rule: No Big Swings

"ALK burn" from a rapid alkalinity jump can kill coral tissue overnight. Adjust KH slowly — about 1 dKH per day — and keep it consistent.

Frequently Asked Questions

Most reef keepers target 8–12 dKH. This range supplies carbonates for coral growth while keeping pH stable. Consistency is more important than chasing a single exact value.

KH (carbonate hardness / alkalinity) measures carbonate and bicarbonate buffering capacity, which stabilises pH. GH (general hardness) measures dissolved calcium and magnesium. Marine and reef care focuses on KH; freshwater care often focuses on GH.

Alkalinity is the water's buffering capacity. A higher KH resists pH changes by neutralising the acids produced by respiration and biological filtration, preventing dangerous pH crashes.

Not fully — alkalinity and pH are linked, so raising KH will nudge pH up too. The safe approach is to raise both slowly (no more than about 1 dKH per day) and let pH follow, rather than trying to force one up while holding the other perfectly still.

Related water chemistry guides

KH is one piece of overall water chemistry — these cover the rest.