What Is Alkalinity and Why Does It Matter for Reef Tanks?
Quick Answer
Alkalinity is your reef tank's pH buffering capacity, measured in dKH. It stabilizes pH and provides carbonate/bicarbonate ions that corals need to build their calcium carbonate skeletons. Most reef tanks should maintain 7-11 dKH.
Alkalinity is your reef tank's pH buffering capacity—it measures the water's ability to resist pH changes and provides essential carbonate and bicarbonate ions that corals use to build their calcium carbonate skeletons.
Understanding Alkalinity
While pH tells you how acidic or basic your water is at a moment in time, alkalinity measures the water's resistance to pH swings. Think of alkalinity as your tank's pH "shock absorber." Water with proper alkalinity can handle the natural acids produced by biological processes without crashing.
Alkalinity is measured in degrees of carbonate hardness (dKH), parts per million (ppm), or milliequivalents per liter (meq/L). Most hobbyists use dKH.
Ideal Alkalinity Levels
| Tank Type | Recommended dKH |
|---|---|
| Soft coral/fish-only | 7-9 dKH |
| Mixed reef (LPS/softies) | 8-10 dKH |
| SPS-dominant reef | 7-9 dKH |
| Natural seawater | ~7 dKH |
Consistency matters more than hitting an exact number. Corals adapt to your tank's stable level—sudden swings cause stress, bleaching, or tissue loss.
Why Alkalinity Matters
- Coral growth: Corals extract carbonate ions from the water to build their skeletons. Low alkalinity = slow growth.
- pH stability: Alkalinity buffers against the pH drops caused by CO2 accumulation and biological waste.
- Calcium relationship: Alkalinity and calcium are consumed together during calcification—you can't maintain one without the other.
What Causes Alkalinity Drops?
- Coral uptake: Growing corals constantly consume alkalinity
- Acid production: Bacteria and biological processes produce acids
- Evaporation effects: While evaporation concentrates minerals, buffering capacity can still shift
- Insufficient water changes: Fresh saltwater replenishes alkalinity
How to Test and Maintain Alkalinity
- Test regularly: Weekly for stable tanks, daily when adjusting. Track trends with the ReefBay app.
- Use quality salt: Most reef salts mix to 8-11 dKH—know your salt's baseline.
- Dose as needed: Use a two-part solution (alkalinity + calcium) or a calcium reactor for demanding tanks.
- Make gradual changes: Adjust no more than 1 dKH per day to avoid shocking corals.
Alkalinity, Calcium, and Magnesium: The Big Three
These three parameters work together. If magnesium is too low (<1250 ppm), it becomes difficult to maintain proper calcium and alkalinity—they'll keep falling no matter how much you dose. Always check magnesium if you're struggling with the other two.
For SPS-dominant tanks, learn more about maintaining all three parameters in our Dosing 101 Guide.
Bottom Line
Stable alkalinity between 7-11 dKH keeps your pH steady and gives corals the building blocks they need to grow. Test weekly, track your trends, and make slow adjustments. Your corals will thank you with vibrant growth and color.
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