Why alkalinity matters so much
Alkalinity (measured in dKH) is the carbonate corals use to build skeleton. Target is 7–11 dKH, but the exact number matters less than holding it steady — corals adapt to a stable 8.5 far better than a number bouncing between 7 and 10. A swing of more than ~1 dKH in a day is enough to stress sensitive corals.
What makes it swing
- Coral demand. As corals grow, they consume more alkalinity — a level that held last month now falls daily. Demand rises over time; your dosing has to keep up.
- Inconsistent dosing. Dosing a big slug once a day (or forgetting a day) creates a saw-tooth. Corals feel the peaks and valleys.
- Low magnesium. Magnesium keeps calcium and alkalinity in solution. When it's low, alk and calcium fight each other and won't both stay put — check magnesium first when the big three won't settle.
- Precipitation. Dosing alk and calcium at the same spot/time, or a pH spike, can precipitate them out — you dose but the number drops.
How to hold it steady
- Test 2–3× per week until you know your tank's daily consumption. You can't dose for demand you haven't measured.
- Dose consistently. Split the daily dose into smaller amounts — a dosing pump spread across the day beats one manual slug.
- Match dosing to consumption. Once you know how much dKH the tank burns per day, replace exactly that. Increase gradually as corals grow.
- Keep magnesium in range (1250–1350 ppm) so alk and calcium stay in solution.
- Never overcorrect. If alk is low, raise it slowly — a fast jump is its own kind of swing. Move ~0.5 dKH per day, not several at once.
Stability comes from watching the trend
Alkalinity is a moving target because coral demand keeps changing — the only way to stay ahead is to track it. Log every dKH test in TankMate and the trend line shows you exactly how fast your tank is consuming it, so you can dial dosing in before corals ever feel a swing. It's free on Google Play.