pH is one of the most misunderstood reef numbers. It affects coral growth, fish stress, and alkalinity stability. The good news is that most pH problems have simple causes.
What pH means in a reef tank
pH measures how acidic or basic your water is. Reef tanks run best in a narrow range. Aim for 7.8 to 8.4, with stability as the main goal. Most healthy reefs sit near 8.0 to 8.3.
pH moves each day due to gas exchange. It drops at night when photosynthesis stops. It rises during the light cycle as algae and corals consume CO2. A daily swing of 0.1 to 0.2 is common.
pH is tied to alkalinity and CO2. Alkalinity buffers pH changes. CO2 pushes pH down when it builds up indoors. This is why a tank can have “good alkalinity” but low pH.
Use pH as a trend, not a trophy number. Chasing 8.4 can cause rapid swings. Corals hate swings more than slightly low pH. For a deeper chemistry refresher, see reef alkalinity basics.
- Target pH: 7.8–8.4, with 8.0–8.3 typical
- Daily swing: 0.1–0.2 is normal
- Pair pH checks with alkalinity: 7–11 dKH is common
How to test pH and read the results
Start with reliable testing. A calibrated probe is best for trends. Calibrate monthly with pH 7.00 and 10.00 fluids. Rinse the probe with RO/DI water after each use.
Liquid pH kits work, but they are coarse. They can miss small swings. If you use a kit, test at the same time daily. Test once near lights-on and once near lights-off.
Interpret pH with other numbers. Measure alkalinity, calcium, and magnesium weekly. Good starting targets are 8–9 dKH, 420–450 ppm calcium, and 1300–1400 ppm magnesium. Stable salinity at 1.025–1.026 matters too.
Watch for false readings. Dirty probe tips drift low. Air bubbles around a probe cause jumps. Old test reagents fade and shift colors. If pH seems “wrong,” verify with a second method before you act.
- Calibrate probes every 4 weeks, or sooner if readings drift
- Log pH with time and light status to see the daily curve
- Confirm odd results with a second kit or a friend’s meter
Fixing low pH without causing swings
Most low pH issues come from excess indoor CO2. A common case is a winter reef in a closed house. pH sits at 7.75 all day, even with 9 dKH alkalinity. The fix is better gas exchange, not more additives.
Improve aeration first. Point a powerhead at the surface. Clean salt creep from overflow teeth. Increase skimmer air draw if possible. If your skimmer has an air line, route it to fresh outdoor air.
A refugium can help by shifting the pH low point. Run the refugium light on a reverse schedule. This keeps photosynthesis going at night. It reduces the overnight pH dip in many mixed reefs.
Use additives carefully. Kalkwasser raises pH and supports calcium and alkalinity. Start slow, like 0.5 to 1.0% of tank volume per day in top-off. Stop if pH rises above 8.45. For dosing basics, review kalkwasser dosing guide and nutrient balance tips.
- Fix CO2 first: surface agitation, skimmer air, and fresh air
- Change pH slowly: avoid moves over 0.2 in 24 hours
- Do not “buffer” pH daily without testing alkalinity
High pH is less common but still possible. It often comes from overdosing kalkwasser or high-alk supplements. If pH hits 8.6, stop dosing and increase aeration. Check alkalinity right away and correct it slowly.
Conclusion
Reef tank pH basics come down to stability and CO2 control. Track trends, calibrate tools, and fix root causes first. With steady alkalinity and good gas exchange, pH usually falls into place.
Sources: Randy Holmes-Farley, “The Relationship Between Alkalinity and pH” (reef chemistry article); Randy Holmes-Farley, “Solving Calcium and Alkalinity Problems” (reef chemistry article); Borneman, E. H., Aquarium Corals (husbandry reference).
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