pH reflects the balance between dissolved CO₂ and alkalinity. Most reef tanks run 7.8–8.4, with a natural daily swing as photosynthesis consumes CO₂ during the day and respiration adds it at night.
Why low pH happens
- High indoor CO₂ (tight homes in winter)
- Low alkalinity (low alk)
- Heavy bioload and low gas exchange
Ways to raise pH
| Method | Effect |
|---|---|
| Improve surface agitation / fresh air to skimmer intake | Lowers CO₂ |
| CO₂ scrubber on skimmer air intake | Can raise pH 0.1–0.3 |
| Kalkwasser via ATO | Raises pH while adding Ca and alk |
| Refugium on reverse light cycle | Reduces overnight drop |
| Soda ash for alk dosing | Raises pH more than bicarbonate |
Chemistry context: ocean acidification guide and Randy Holmes-Farley, "A Simplified Guide to the Relationship Between Calcium, Alkalinity, Magnesium and pH," Reefkeeping Magazine.
Sources
FAQ
Is 7.9 pH bad? Usually acceptable if stable, but higher pH within the normal range can support calcification.
Should I chase pH with buffers? Fix CO₂ and alkalinity first; avoid repeated pH buffers.