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Alkalinity (KH) Calculator for Reef Tanks
Alkalinity (KH) Calculator for Reef Tanks — Baking Soda vs Washing Soda
Alkalinity — measured as KH (carbonate hardness) in dKH — is arguably the single most critical parameter in a reef aquarium. It is the fuel corals use to build their calcium carbonate skeletons, and it buffers the tank's pH against acidic swings during the night. Natural seawater has an alkalinity of approximately 7–8 dKH; most reef tanks target 8–12 dKH depending on coral type (LPS tanks often run lower, SPS tanks higher). Alkalinity is consumed rapidly in a coral-dense system — a thriving SPS tank can consume 1–2 dKH per day. When alkalinity crashes, coral tissue is the first casualty: RTN (rapid tissue necrosis) and STN are almost always preceded by sudden KH drops. This calculator tells you exactly how many grams of baking soda (sodium bicarbonate) or washing soda (sodium carbonate) to add to recover your KH to the target level. Both compounds work; washing soda is more concentrated but requires more careful dosing.
How to Use This Calculator
1
Test KH with a reliable test kit (Hanna HI755, Salifert, or ICP). Note the current value in dKH.
2
Enter your target KH. For mixed reef: 8–10 dKH. SPS-dominant: 9–11 dKH. LPS-only: 7–9 dKH.
3
Enter your water volume in liters.
4
Choose baking soda for gentle corrections (up to 2 dKH). Use washing soda if you need a larger correction with less powder.
5
Dissolve completely in 500 mL warm RODI water. Never raise KH more than 1–2 dKH per day to avoid stressing corals.
Need reef supplements?
We stock Brightwell Aquatics, Two Little Fishies, and premium additives — shipped with your corals.
Frequently Asked Questions
What KH level is best for a reef tank?
Natural seawater sits at about 7–8 dKH. Most mixed reefs target 8–10 dKH. SPS-dominant tanks often run 9–11 dKH to fuel rapid skeletal growth. LPS corals are more tolerant and can thrive at 7–9 dKH. Stability matters more than exact number — swings of more than 0.5 dKH per day cause more damage than a 'suboptimal' steady value.
How quickly does KH drop in a reef tank?
Consumption depends entirely on coral density and growth rate. A lightly stocked LPS tank might lose 0.2–0.5 dKH per day. A packed SPS system can consume 1.5–2+ dKH daily. Measure your daily drawdown over 3–5 days, then set your dosing accordingly.
What is the difference between baking soda and washing soda for KH?
Both raise alkalinity. Baking soda (NaHCO₃) is gentler — it requires about 1.6× more grams than washing soda to raise the same dKH, but the slower CO₂ release makes pH swings less sharp. Washing soda (Na₂CO₃) is more concentrated and faster-acting but can cause temporary pH spikes and should be used more carefully.
Can I use kalkwasser to raise KH?
Kalkwasser (calcium hydroxide solution) raises both calcium and alkalinity simultaneously, but it elevates pH significantly (to 12+), meaning it must be dripped slowly. It's excellent for top-off water and gentle long-term maintenance but not ideal for rapid KH corrections.
Why did my KH crash suddenly?
Sudden KH crashes are usually caused by a bacterial bloom (bacteria consume alkalinity rapidly), a new large coral placement, a dosing pump failure, or a very fast-growing SPS colony overtaking your dosing rate. Always investigate the root cause before just re-dosing buffer.
