Stable KH in an SPS reef: the key to hard corals
Why alkalinity stability matters more than the exact value, and how to raise KH without burning your acroporas.
June 15, 2026
If you keep SPS corals (acroporas, montiporas and company), there’s one parameter that rules over almost all the others: alkalinity (KH). Not so much its absolute value, but its stability.
Why KH is so critical
KH measures the water’s buffering capacity: how many carbonates and bicarbonates are available for the coral to build its skeleton. When it swings from one day to the next, the coral suffers stress, loses color and, in severe cases, the dreaded STN/RTN (tissue necrosis) appears.
An SPS reef prefers a KH of 7.5 rock-solid over one that bounces between 7 and 9.
The target range
- KH: 7.5–8.5 dKH (kept stable)
- Ca: 410–440 ppm
- Mg: 1300–1400 ppm
The trick is that these three move in step. If magnesium is low, calcium and KH won’t hold no matter how much you dose. That’s why, before touching Ca or KH, check the Mg.
The golden rule when raising KH
Never raise the KH more than 1 dKH per day. If your test reads 6.5 and your target is 8, don’t add the entire dose at once: spread it over several days. A sharp rise in alkalinity is one of the fastest ways to lose a colony.
The App Acuarios calculator applies this rule automatically: if the correction exceeds 1 dKH/day, it spreads the dose across the necessary days and warns you.
Recommended routine
- Always test KH at the same time of day (alkalinity varies throughout the day).
- Log the value in the app and let it calculate your daily consumption.
- Dose the consumption consistently (better spread out than all at once).
- Check Mg once a week.
Stability isn’t glamorous, but it’s what separates a reef that survives from one that grows and bursts with color.