Parameter Reference
AlkalinitydKH
Alkalinity (often called KH or dKH in the reef hobby) measures the water's capacity to neutralize acid — in reef tanks, mostly bicarbonate (HCO₃⁻) and carbonate (CO₃²⁻). It's the carbon source coral skeletons are built from, and the parameter that moves fastest in either direction. Coral judges your tank by how stable Alk is, not by where it sits inside the safe band.
Target ranges
| Beginner-safe | 7–11 dKH |
| Optimal (consensus) | 7–9 dKH |
| Triton method target | 7–8 dKH |
| Natural seawater | ~6.5–7 dKH (2.33 meq/L) |
| Unit conversions | 1 dKH = 0.357 meq/L = 17.86 ppm CaCO₃ |
Why it matters
Coral skeletons are CaCO₃. Building a skeleton requires both Ca²⁺ and CO₃²⁻ ions in solution — the carbonate side comes from alkalinity. If alk runs low, coral can have plenty of calcium and still fail to calcify. If it runs high, the same Ca²⁺ + CO₃²⁻ combination precipitates spontaneously, leaving you with a "snow storm" of CaCO₃ in the water column and burned SPS tips.
The rate of change matters more than the destination. A tank running stable at 6.5 dKH for a year is healthier than one bouncing between 8 and 11 every week.
Symptoms of drift
Too low
Below 6 dKH: slowed calcification, thinner skeletons, coral bleaching. Below 5: calcification effectively stops (Randy). The classic early symptom is recession at the base of SPS colonies.
Too high
Above 12 dKH: SPS tissue recession ("burnt tips"), Ca/Mg depletion as alk pulls them into precipitation, and rapid pH climb. Snowstorm precipitation risk if Ca is also dosed high.
Testing
Salifert and Red Sea KH Pro titration kits are the hobbyist standard — ±0.2 dKH if you read the syringe carefully. Hanna HI772 colorimeter is faster and removes user error but runs about $50 and the reagent costs add up. ATI and Triton ICP labs report alkalinity but with a multi-day turnaround that defeats the purpose for active dosing.
Test once a week minimum. Test before and after any dosing change. Test once a day if you're commissioning a new dosing pump.
KH for reef tank — target, conversion, and how to raise or lower it
"KH for reef tank" is the most-searched phrasing for this parameter, and it's functionally the same as dKH. KH stands for carbonate hardness; dKH adds the German degree unit. The titration kits all report dKH; the test result you write in your log is the dKH. The two terms are interchangeable in reef hobby use.
Target band for a reef tank:
- SPS-dominant, Triton method, or stability-focused: 7–9 dKH.
- Mixed reef, Red Sea Coral Pro setup: 8–11 dKH. Faster calcification, less margin for swings.
- Natural seawater for comparison: ~6.5–7 dKH (2.33 meq/L).
To raise KH: dose sodium carbonate (soda ash) or sodium bicarbonate (baked or unbaked baking soda). The fastest path is the alkalinity dosing calculator — enter current dKH, target dKH, tank volume, and pick BRS Soda Ash, Randy Recipe 1, AFR, or Red Sea Foundation B. Cap corrections at 1.4 dKH per day to avoid stressing coral.
To lower KH: water change with a lower-dKH salt mix is the only safe option. Acid-dosing techniques exist but carry pH-crash risk. Stop dosing and wait — coral consumption will drop dKH at 0.5–2 dKH/day depending on bioload.
The two-part chemistry behind it is covered in the two-part dosing guide. The aquarium-wide nitrogen cycle and KH interaction (NO₃ buildup acidifies the water and consumes KH) is in the nitrogen cycle guide.
FAQ
- Why do BRS, Triton, and Randy give different targets?
- Different opinions on whether to match natural seawater (6.5–7 dKH) or run elevated for faster calcification (8–11 dKH). Elevated alk grows SPS faster but is less forgiving of swings. Most modern reefers compromise around 8 dKH.
- Is dKH the same as KH?
- In aquarium use, yes — dKH (German degrees of carbonate hardness) is the unit everyone reports, despite KH technically meaning carbonate hardness which is a slightly different thing. Don't worry about the distinction.
Sources & references
- 01Randy Holmes-Farley — Optimal Parameters for a Coral Reef Aquariumhttps://www.reef2reef.com/threads/optimal-parameters-for-a-coral-reef-aquarium-by-randy-holmes-farley.173563/
- 02Randy Holmes-Farley — Reefkeeping Recipe #2 articlehttps://reefkeeping.com/issues/2006-02/rhf/index.php
- 03Triton — CoreChem product / method targetshttps://www.triton.de/en/products/corechem
- 04Global Seafood Alliance — Typical chemical characteristics of full-strength seawaterhttps://www.globalseafood.org/advocate/typical-chemical-characteristics-of-full-strength-seawater/
- 05Hanna Instruments — HI772 Marine Alkalinity Checkerhttps://hannainst.com/marine-alkalinity-dkh-checkerr-hc-hi772.html
Last reviewed