This reef tank alkalinity calculator calculates the dose of alkalinity supplement needed to raise your aquarium from its current dKH level to your target. Supports sodium bicarbonate (baking soda), two-part liquid supplements, and kalkwasser dosing methods.
Alkalinity Dose Calculator
Reef Chemistry Reference
| Parameter | Mixed Reef | SPS-Dominant | LPS-Dominant |
|---|---|---|---|
| Alkalinity (dKH) | 8–10 | 7–9 | 8–11 |
| Calcium (ppm) | 400–450 | 420–450 | 380–430 |
| Magnesium (ppm) | 1260–1350 | 1280–1400 | 1260–1350 |
| pH | 8.1–8.3 | 8.1–8.3 | 8.0–8.3 |
How to Raise Alkalinity in a Reef Tank
Alkalinity (also called carbonate hardness or dKH) is the primary buffer that keeps reef aquarium pH stable. Corals and coralline algae consume alkalinity as they build calcium carbonate skeletons. In an active reef, alkalinity can drop 0.5–4 dKH per day depending on coral mass and growth rate.
Why Stability Matters More Than Target Numbers
Swings of more than 1–2 dKH within 24 hours can trigger bleaching, tissue necrosis (RTN/STN), or polyp recession in SPS corals. Before starting any dosing regimen, test alkalinity at the same time each day for 3–5 days to understand your tank's daily consumption. Then dial in your dose to match consumption rather than chasing a target number.
Sodium Bicarbonate — Cheapest Method
Baking soda (NaHCO₃) raises alkalinity without significantly changing calcium. To avoid pH shock, pre-dissolve it in a cup of tank water and add slowly near high-flow areas. For large adjustments, split the dose over 24–48 hours. Food-grade baking soda from a grocery store works perfectly — no need to buy the "aquarium version" at markup.
Two-Part Dosing — Best for Consistency
Two-part systems provide both alkalinity and calcium as separate liquid concentrates you dose in equal parts. They're easy to automate with a dosing pump, highly predictable, and maintain the balance between alkalinity and calcium. They're more expensive than baking soda per unit of alkalinity but far more convenient for maintaining a stable reef long-term.
FAQ
Is this reef tank alkalinity calculator free?
Yes, completely free with no signup required.
What is the ideal alkalinity level for a reef tank?
Most reef keepers target 8–12 dKH (2.86–4.29 meq/L). SPS-dominant tanks often target 7–9 dKH. Mixed reefs work well at 8–10 dKH. LPS-dominant tanks tolerate 8–11 dKH. Stability matters more than hitting a specific number — swings of more than 1–2 dKH per day can cause coral bleaching or RTN.
How much does alkalinity drop per day in a reef tank?
Consumption varies widely. A lightly stocked mixed reef may drop 0.5–1 dKH/day. A heavily stocked SPS tank can drop 2–4 dKH/day. The only way to know your tank's consumption is to test alkalinity at the same time daily for a week without dosing. That gives you your baseline drop rate for two-part or kalkwasser maintenance dosing.
Can I raise alkalinity too fast?
Yes. Raising alkalinity more than 1–2 dKH per day can precipitate calcium out of the water (calcium carbonate snow), lower pH suddenly, and stress or kill corals. If you need to raise alkalinity by more than 2 dKH, do it over 2–3 days in stages. Always test before each dose.
What is the difference between two-part dosing and kalkwasser?
Two-part systems dose alkalinity and calcium separately as liquid concentrates — precise and controllable. Kalkwasser (calcium hydroxide) doses both calcium and alkalinity together as a saturated solution, and also raises pH. Kalkwasser is more economical for tanks with moderate demand but requires more careful handling. Calcium reactors and dosing pumps are the professional solution for high-demand SPS tanks.