Corals adapt to a range of alkalinity levels, but they handle rapid change poorly. A swing of more than about 1 dKH within a day is a common trigger for tissue loss in SPS. Stabilizing alk is usually the single most valuable thing you can do for a struggling reef.
What counts as a swing?
| Daily change | Risk |
|---|---|
| < 0.3 dKH | Excellent stability |
| 0.3–0.5 dKH | Fine for most tanks |
| 0.5–1 dKH | Watch SPS closely |
| > 1 dKH | High risk of STN/RTN |
Common causes
- Once-a-day dosing of a large amount. Split into many small doses with a dosing pump (doser guide).
- Kalkwasser via ATO on hot days, when evaporation (and kalk dose) spikes.
- Water changes with salt that mixes at a very different alk from the tank.
- Growing demand: as corals grow, a fixed dose becomes too little and alk drifts down, then a big correction causes a spike.
- Test error: an inconsistent kit makes it look like a swing. Retest before reacting.
Plan to stabilize alk
- Test at the same time every day for one week to learn your consumption.
- Set your daily alk dose to match consumption, split over 12–24 doses.
- Adjust dose by no more than 10% at a time.
- Match new saltwater alk within ~0.5 dKH of the tank.
- Consider an automated alk monitor or log tests in a tracker to see trends.
Sources
FAQ
Is a 0.5 dKH swing bad? Most tanks tolerate it; steadier is better for SPS.
Why does alk drop overnight? Corals keep calcifying at night, and if dosing stops overnight alk falls. Spread doses across 24 hours.