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Low alkalinity is the most common reef chemistry problem, and raising it too fast is the most common way to make it worse. The rule: raise alkalinity by no more than about 1 dKH per day, then retest.
What alkalinity should a reef tank be?
Most reefs run 7–9 dKH. Pick a target in that range (many people aim for 8–8.5) and then keep it stable. Stability matters more than the exact number.
Step 1: Test accurately
Use a reliable kit (see our best reef test kits guide). Test at the same time of day, because alkalinity drifts as the lights run.
Products: Hanna HI772 Salifert Alk
Step 2: Choose your alkalinity supplement
| Option | Good for | Notes |
|---|---|---|
| Reef-grade sodium bicarbonate (e.g., the alkalinity part of a 2-part kit, sold dry) | One-off corrections | About 30 mg per liter raises 1 dKH (≈0.11 g per US gallon). Slightly lowers pH. |
| Reef-grade soda ash (sodium carbonate) | Correction that also raises pH | Stronger per gram. Dose slowly into high flow. |
| 2-part (BRS Pharma, Red Sea Foundation, Fritz RPM) | Daily maintenance of alk and Ca together | Balanced, so calcium stays in line. Ideal with a dosing pump. |
| Kalkwasser | Low-demand tanks, pH boost | Dosed through the ATO. Limited by evaporation rate. |
Products: BRS Pharma 2-part Red Sea Foundation B (Alk) Fritz RPM Liquid Alk
Step 3: Work out the dose
Dose depends on net water volume (not the tank's nominal size: subtract rock and sand, add the sump) and on how far you need to go. Type your test result and target into the Coral Compass dosing calculator. It splits the dose so you never exceed ~1 dKH per day.
Step 4: Dose into high flow, then retest
Dissolve powder in RO/DI first, then add it slowly near a return or powerhead, away from corals. Retest after a few hours. Once you're at target, measure your daily consumption (how much alk drops per day) and dose that amount daily, ideally with a dosing pump.
Products: Dosing pump (e.g., Kamoer or Jebao)
For the full chemistry of how alkalinity, calcium and magnesium interact, read our Big 3 guide.
Common mistakes
- Jumping 2–3 dKH in a day. This causes stress and RTN in SPS.
- Dosing alkalinity and calcium at the same spot at the same time, which causes precipitation ("snow").
- Ignoring magnesium. Low Mg makes alk and Ca hard to hold.
- Chasing a single bad test. Retest before you correct.
FAQ
How long does it take for alkalinity to rise? Liquid and dissolved doses mix within about 30–60 minutes in a well-circulated tank.
Can I use grocery-store baking soda? We recommend reef-grade sodium bicarbonate or a 2-part alkalinity product instead. It's made for aquarium use and sold with consistent purity, so your doses match the calculator.