Why it matters
Low calcium dissolves pool plaster and starves coral. High calcium scales heater elements and clouds pool water.
Ideal ranges
| Water type | Ideal | Acceptable |
|---|---|---|
| Reef | 400 to 450 ppm | — |
| African cichlid | 200+ ppm | — |
| Pool (plaster) | 200 to 400 ppm | — |
| Pool (vinyl/fiberglass) | 150 to 250 ppm | — |
| Spa | 150 to 250 ppm | — |
How to test
- 1
Reef: Salifert or Hanna Calcium Checker
Hanna gives a number to within 5 ppm in 60 seconds.
- 2
Pool / spa: drop kit or strip
Strips are acceptable for monthly checks. Drop kit when starting up.
How to correct low calcium hardness
Reef: dose calcium chloride (2-part)
Pair with alkalinity dose to keep balance.
Pool: add calcium chloride flakes
1 lb per 10,000 gallons raises CH by ~10 ppm. Pre-dissolve in a bucket of pool water.
How to correct high calcium hardness
Partial drain and refill with low-CH water
Only practical fix for pools. RO water for reef.
Use a scale inhibitor
Buys time but does not lower CH. Helps prevent staining.
Common causes
- •Hard tap water
- •Calcium chloride pool startup
- •Coral and coralline growth depleting (reef)
- •Pool plaster curing leaches calcium
FAQs
Why is my CH rising even though I do not add calcium?
Evaporation concentrates calcium while top-off water keeps adding more. Use RO/DI for top-off in reef and pool.