Natural seawater sits around 6.5–7.5 dKH (roughly 2.3–2.6 meq/L). Many successful reefs, especially SPS systems that run lean on nutrients, deliberately run alkalinity at or near that natural level instead of the 9–11 dKH that was fashionable a decade ago. This post explains why, when lower alk makes sense, and how to get there without crashing your corals.
Why run lower alkalinity?
- Wider safety margin with low nutrients. Hobbyist experience, summarised widely on Reef2Reef and BRStv, is that high alk (10+ dKH) combined with very low nitrate and phosphate is a common recipe for burnt tips on SPS. Lower alk is more forgiving when nutrients bottom out. See our ULNS research guide.
- Less abiotic precipitation. The higher alk and calcium are pushed above natural seawater, the more calcium carbonate wants to precipitate on heaters, pumps and sand (Randy Holmes-Farley, "A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH," Reefkeeping Magazine).
- Swings hurt less in absolute terms. What matters is stability; lower targets mean a smaller daily dose and a smaller error if a doser misfires.
Why low alk can go wrong
Coral calcification slows as carbonate availability falls. Below about 6 dKH growth often slows noticeably and pH tends to drop. Corals adapted to 9 dKH can show stress if you drop them to 7 in a few days. Lower alk is a target you migrate to, not a correction you make overnight.
Who should consider 7–7.5 dKH?
| Tank | Suggested alk | Why |
|---|---|---|
| SPS dominant, low nutrients (NO3 < 2 ppm, PO4 < 0.03 ppm) | 7–7.5 dKH | Reduces tip burn risk |
| Mixed reef, moderate nutrients | 7.5–8.5 dKH | Balanced growth and safety |
| Heavily fed tank, NO3 5–15 ppm | 8–9 dKH | Nutrients support higher alk |
| Softies / LPS | 7–9 dKH | Stability matters more than the number |
How to lower alkalinity safely
- Test twice with a reliable kit to confirm the starting value (see Hanna checkers vs titration kits).
- Reduce the alkalinity dose, don't add acid. Cut your daily alk dose by 10–20% and let coral uptake bring the number down.
- Aim for no more than ~0.5 dKH drop per day, ideally slower. Over a week or two is fine.
- Once at target, re-measure daily consumption and set the doser to match.
- Check your salt mix. Some mixes come out at 9–12 dKH; big water changes will push alk back up. Match new saltwater to the tank or change smaller volumes (see salt mix guide).
Signs your alk is too low
- Falling pH, especially overnight (below ~7.8)
- Slower growth and pale growth tips on SPS
- Coralline algae stalls
- Alk drifting down despite your normal dose (consumption rose, time to increase dose)
If your alk is under 6 dKH, raise it at no more than 1 dKH per day using reef-grade sodium bicarbonate or 2-part (guide: How to raise alkalinity safely).
Sources
FAQ
Is 7 dKH too low for SPS? No. 7 dKH is close to natural seawater and many top SPS tanks run there. Hold it stable and keep nitrate and phosphate above zero.
Can I use acid to lower alkalinity? It's not recommended for hobbyists. Reducing dosing and letting corals consume it is far safer.
Does lower alk mean lower calcium? Not necessarily. Keep calcium near 400–440 ppm. Calcium and alk are consumed in a fixed ratio (~7.1 ppm Ca per 1 dKH), so ratios stay balanced if you use 2-part.