Glossary
Alkalinity (KH)
How much carbonate the water carries, and the single number stony corals care most about.
Measured in dKH. The absolute value matters less than its stability: a coral held at 8.4 for months will sulk if it swings to 9.5, even though 9.5 is a perfectly good number on its own.
Test at the same time of day, always. Alkalinity falls through the photoperiod as corals build skeleton and rises again overnight, so a morning reading and an evening reading are answers to two different questions — and comparing them is how people talk themselves into a correction they did not need.
Related terms
- Coralline algaeThe pink and purple crust on established rock — a sign the system is mature.
- SPSSmall-polyp stony corals — the demanding, fast-growing, colour-shifting end of the hobby.
- Polyp extensionHow far the animal opens its tentacles — the fastest read on whether it is comfortable.