More reef tanks are lost to alkalinity swings than to any pest. Not to a crash from a broken heater, not to a pest outbreak — to a dKH number that moved too far, too fast, usually while someone was trying to fix it.
Why alkalinity is the parameter that bites
Alkalinity is your system’s buffering capacity: its resistance to pH change. Corals consume it continuously to build skeleton, which means in any tank with real growth alkalinity is always falling. It is the one major parameter that is actively being drawn down every hour of every day.
That has two consequences. Alkalinity needs replacing on a schedule, and because it interacts directly with calcium and pH, a large sudden change hits coral tissue harder than an equivalently sized change in almost anything else.
What a swing does to corals
A rapid drop typically shows up as SPS tips going pale, polyps staying in, and growth stalling. Severe drops cause tissue recession from the base — the classic “burnt tips” and rapid tissue necrosis stories usually have an alkalinity event somewhere in them.
A rapid rise is worse and faster. Overshooting alkalinity burns tissue, and if calcium is high at the same time you can trigger precipitation — a cloudy tank, a coating of white on heaters and pumps, and a simultaneous crash in both calcium and alkalinity as they drop out of solution together.
The practical threshold most reefers work to: do not move alkalinity more than about 1 dKH in a 24-hour period. If you are 2 dKH low, that is a two-day correction, not an afternoon one.
The four causes, in order of how often they are the culprit
1. Inconsistent dosing. Dosing by hand when you remember it produces a sawtooth — high after, low before. A doser splitting the daily requirement across many small doses produces a flat line. If you dose manually and your corals are moody, this is your answer.
2. Growth outrunning the schedule. A tank that consumed 5 dKH a week last quarter may consume 8 this quarter. Consumption rises as coral mass rises, and a dosing amount that was correct three months ago is now too small. Re-test consumption periodically.
3. Panic correction. You test, get a low reading, dose a big correction, and put the tank through a bigger swing than the original deficit ever caused. This is the most common way alkalinity actually kills corals: the treatment, not the disease.
4. Bad test data. Expired reagent, sloppy technique, or a kit read in poor light. Before you correct anything that surprises you, retest. Emergency corrections based on wrong numbers cause real emergencies.
The habits that prevent all of it
- Pick a value you can hold and hold it. A steady 7.8 dKH beats a tank bounced between 7 and 9 chasing 8.5. Corals adapt to a consistent number.
- Automate if you can. A cheap doser is the single best-value equipment purchase in reefkeeping, and it removes the most common failure mode entirely.
- Log every test. A number in range but trending down for three weeks is information. A single reading in isolation is almost useless.
- Correct slowly, always. There is no alkalinity deficit so urgent that a fast correction is the right answer.
Keep the targets in front of you
Grab our free reef water chemistry quick-reference chart — every target range on a single printable page, no charge.
For what each parameter actually does, how to correct safely, how to calculate your consumption, and how to spot a bad test before you act on it, the Reef Water Parameters Quick Reference Guide is an instant PDF download.