Reef pH normally sits between 7.8 and 8.5, and 8.1–8.3 is better if you can manage it. It’s lowest first thing in the morning and highest in the afternoon. Low pH usually comes from low alkalinity or carbon dioxide-heavy indoor air, so check alkalinity first, then improve aeration.
The normal daily swing
Corals and algae use carbon dioxide while the lights are on, which lifts pH. At night they release it, and pH falls. Bulk Reef Supply describes pH as lowest in the morning and rising through the day. That’s normal, provided it stays in range.
So test pH at the same time each day, or note both a lights-on and a lights-off reading. Comparing a morning reading with an evening one tells you nothing.
Why pH runs low
Bulk Reef Supply lists three main causes.
- Low alkalinity. Alkalinity buffers pH. Fix this first; see alkalinity.
- Carbon dioxide-heavy indoor air. Well-sealed, occupied rooms build up CO₂, and a skimmer pulls that air straight into the water. This is common in UK homes in winter, with windows shut.
- Night time. The overnight dip is deepest where the room air is stale and the tank has little surface movement.
How to raise it
Work through these in order.
- Check alkalinity. If it’s low, raise it gradually. If it’s in range and pH is still low, look elsewhere.
- More surface movement and aeration. Point a wavemaker at the surface, or add an airstone to the sump.
- Fresher air. If a window can open near the skimmer intake, test whether pH rises. Not practical all year.
- A CO₂ scrubber. Feeds the skimmer air through media that removes carbon dioxide. Bulk Reef Supply covers these.
- A refugium or algae reactor. Photosynthesis uses CO₂, and a light timed opposite the main tank can soften the overnight dip.
Measuring pH
Colour pH kits are hard to read precisely. A probe or meter is better, but it needs calibrating regularly with fresh buffer solutions; Bulk Reef Supply lists uncalibrated probes as a common testing mistake. Note the time with every reading.
The Companion’s test log doesn’t have a pH field yet. Until it does, add pH readings to the note on each test, with the time.
Common questions
Is pH 7.8 OK for a reef tank?
It’s at the bottom of the usual range. If that’s your overnight low, many tanks are fine. If it’s your daytime reading, look at alkalinity and air.
Does baking soda raise pH?
No; it slightly lowers it while raising alkalinity. Soda ash and kalkwasser raise pH.
Should I chase pH with buffers?
No. Fix alkalinity and aeration, then let pH follow.
Sources and limits
Last checked 12 October 2026. Ranges and doses here come from the published sources below and from the chemistry of each additive. They’re starting points for your own tank, not guarantees for any coral.
- Randy Holmes-Farley, Reefkeeping: Reef Aquarium Water Parameters
- Bulk Reef Supply: how to raise pH in your saltwater aquarium
- Bulk Reef Supply: 10 common water testing mistakes