Alkalinity Too High in a Reef Tank

Alkalinity is the most-watched number in reef chemistry because it is consumed fastest and swings hardest. Most tanks run comfortably between 7 and 12 dKH. When alkalinity drifts above your normal point — say toward 13 or 14 dKH — coral tips can burn, polyps stay retracted, and growth stalls. Just as important: how fast it changed often matters more than the absolute number.

What pushes alkalinity up

• Overdosing a buffer or 2-part. The most common cause — a dosing pump set too high, or a manual dose added on top of an automatic one.
• Salt mix. Some salts mix to 9–12 dKH straight out of the bucket; a large water change can spike alk if your tank was already running high.
• Low calcium-demand tank. A young or coral-light system consumes little alkalinity, so a dosing schedule meant for a mature reef overshoots.
• Kalkwasser plus 2-part together without reducing either, stacking two alkalinity sources.

How to correct it safely

The cardinal rule: bring alkalinity down slowly. A safe ceiling is roughly 0.5–1.0 dKH of change per day. The gentlest fix is simply to stop or reduce dosing and let the tank's natural consumption pull alkalinity down on its own over a few days — corals and coralline will draw it back into range without any intervention.

If you need to move faster, perform partial water changes with a salt mix that tests lower than your current tank value; this dilutes alkalinity gradually while topping up other elements. Recheck alkalinity 12–24 hours after each step rather than chasing it within the same hour, since test kits and short-term fluctuation can mislead you.

Why swings are worse than the number

Coral skeletons are deposited at a rate tied to stable alkalinity. A rapid drop or rise — even back toward "ideal" — can cause tissue recession and the dreaded burnt SPS tips. Never crash a 13 dKH tank to 8 dKH in a day. Recalibrate your dosing so the new lower point holds steady, and confirm calcium and magnesium are in range too, since they move together.