Why it matters
Low alkalinity lets pH crash. High alkalinity locks pH high and makes it hard to adjust. Reef tanks need stable alkalinity for coral skeleton growth.
Ideal ranges
| Water type | Ideal | Acceptable |
|---|---|---|
| Freshwater community | 4 to 8 dKH | — |
| African cichlid | 10 to 18 dKH | — |
| Reef | 8 to 11 dKH | — |
| Pool | 80 to 120 ppm | — |
| Spa | 80 to 120 ppm | — |
- Reef: Stability matters more than the exact number. Hold within 0.5 dKH.
How to test
- 1
Aquarium: use Salifert, Hanna, or Red Sea kit
Hanna Checker is fastest. Salifert is most accurate for the price.
- 2
Pool: liquid drop kit or strip
Drop kits are more accurate than strips for TA.
How to correct low alkalinity
Reef: dose 2-part or kalkwasser
Two-part (alk + cal) keeps the ratio in balance. Kalkwasser tops off both alk and pH.
Pool: add baking soda (sodium bicarbonate)
1.5 lb per 10,000 gallons raises TA by ~10 ppm. Spread across the pool, brush in.
Freshwater: crushed coral, aragonite, or Seachem Equilibrium
Slow steady release. Avoid rapid alkalinity swings.
How to correct high alkalinity
Pool: add muriatic acid in small doses
Lowers pH and TA together. Aerate after to bring pH back up while leaving TA lower.
Aquarium: water changes with lower-KH source water
RO/DI mixed with tap water lets you dial in target KH.
Common causes
- •Carbonate substrate dissolving (raises KH)
- •Acidic events lowering KH over time
- •Pool refill water with high TA
- •Coral skeleton growth consuming alk in reef tanks
FAQs
What is the difference between KH and TA?
KH is carbonate hardness, measured in dKH. TA is total alkalinity, measured in ppm CaCO3. Multiply dKH by 17.86 to get ppm.
Why is reef alkalinity so picky?
Coral skeletons are calcium carbonate. Swings of more than 0.5 dKH in a day cause tissue recession (alk burn). Stability beats the perfect number.