Parameter Reference
pH
pH is an output, not a target. It's determined by alkalinity and dissolved CO₂ in a fixed equilibrium. If your tank is stuck at 7.8 with 9 dKH alk, the answer isn't to dose more — it's room-air CO₂. Fix the cause; pH follows.
Target ranges
| Beginner-safe | 7.8–8.5 |
| Optimal (consensus) | 8.1–8.3 |
| Triton target (with Core7) | ~8.0–8.3 |
| Natural seawater (open ocean) | 7.6–8.4, avg 8.1 |
| Bone dissolution risk below | 7.7 |
| Precipitation risk above | 8.55 |
Why it matters
pH controls CaCO₃ solubility equilibrium. Below 7.7, coral skeletons can slowly dissolve. Above 8.55, CaCO₃ precipitates spontaneously, clogging skimmer pumps and dropping fine white particulate everywhere. The 7.9–8.4 sweet spot is where most reef chemistry works without active interference.
Daily pH swings are normal: tanks dip 0.1–0.2 overnight (no light, no photosynthesis, CO₂ builds from respiration) and recover during the day. Swings larger than 0.3 indicate either a CO₂ problem (room air) or a buffer problem (low alk).
Symptoms of drift
Too low
Below 7.8: coral bleaching risk, suppressed calcification. Below 7.7: skeleton/bone dissolution (Randy). Elevated CO₂ is the usual cause — not insufficient alkalinity.
Too high
Above 8.5: kalkwasser overdose indicator, CaCO₃ precipitation in equipment, coral stress from rapid swings. Above 8.6 sustained will start failing skimmer pumps from internal precipitation.
Testing
Hanna HI98127 or Milwaukee SM600 pH pens are accurate enough ($30–60 range). Apex or Trident probes give continuous logging but drift — recalibrate quarterly with 7.0 and 10.0 standards. pH paper test kits are useless at reef precision; ignore them.
Test at the same time every day if you're tracking trends. Tanks with kalkwasser dosing will read 0.1–0.2 higher overnight when kalk is dripping; tanks without buffer will dip the same amount.
FAQ
- How do I raise pH that won't climb?
- Source air CO₂ first. Run airline tubing from outdoors to the skimmer's air intake — most reef tanks pin pH at room CO₂. A CO₂ scrubber on the skimmer line works too. Dosing more alkalinity raises pH only marginally; the equilibrium fights you.
- Should I chase 8.3?
- Not at the cost of stability. A tank at stable 8.0 grows coral better than one swinging 7.9–8.4 trying to hit 8.3. Pick a number your tank holds naturally and let it stay there.
Sources & references
- 01Randy Holmes-Farley — Optimal Parametershttps://www.reef2reef.com/threads/optimal-parameters-for-a-coral-reef-aquarium-by-randy-holmes-farley.173563/
- 02Bulk Reef Supply — Using Kalkwasser (pH 0.20 rule)https://www.bulkreefsupply.com/content/post/using-kalkwasser
- 03Global Seafood Alliance — Typical seawater characteristicshttps://www.globalseafood.org/advocate/typical-chemical-characteristics-of-full-strength-seawater/
- 04UKAPS — Water pH by dKH and CO₂ valueshttps://www.ukaps.org/forum/threads/water-ph-by-dkh-and-co2-values.77203/
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