Reef Alkalinity (dKH) Guide

This guide covers reef alkalinity dKH with practical steps for home aquariums. Alkalinity is the backbone of coral skeleton building — unstable dKH causes more reef losses than any single parasite.

For related reading, see aquarium water chemistry basics, mastering aquarium pH and aquarium filtration.

What Alkalinity Is

Alkalinity measures carbonate and bicarbonate ions available for calcification — reported as dKH or meq/L. Corals and coralline algae consume it continuously.

It buffers pH alongside calcium and magnesium — the trio must move together.

Target Ranges

Most mixed reefs run 7–9 dKH. Some SPS keepers hold 8–9; lower or higher can work if stable. Wild daily swings hurt more than picking 8 vs 9.

New tanks may drift as rock equilibrates — test daily first month on stony coral systems.

Testing Discipline

Test alkalinity at least twice weekly on coral tanks; daily when dosing manually. Calcium and magnesium weekly.

Use consistent test kits — comparing Hanna to Salifert on the same sample reveals method bias.

Dosing and Consumption

Two-part alkalinity and calcium solutions dose separately on timers. Increase dose when consumption rises after adding stony corals.

Never dump large single doses to 'catch up' — spread corrections across hours or days.

Crash Recovery

If alkalinity crashed from overdose or equipment failure, bring levels up slowly with small water changes and conservative dosing while watching pH.

Corals may bleach after severe swings — stability afterward matters more than chasing perfect numbers immediately.