Alkalinity and calcium drive coral growth. They also shape pH stability. If they drift apart, reefs lose color and stop growing.
This guide explains how to balance both. You will get target numbers, dosing steps, and fixes for common mistakes.
What alkalinity and calcium do in a reef tank
Alkalinity is your tank’s buffering capacity. It resists pH swings. It is measured as dKH or meq/L. Most reef tanks run best at 7.5–9.0 dKH.
Calcium is a building block for skeletons. Stony corals and coralline algae use it fast. A solid range is 400–450 ppm. Many tanks sit near 420 ppm.
Corals consume alkalinity and calcium together. The chemical ratio is fixed in calcification. If you only dose one, the other becomes limiting. Growth slows and parameters drift.
Magnesium supports both values. It reduces unwanted precipitation. Keep magnesium at 1250–1400 ppm. Test it monthly or after big corrections.
- Target range: 7.5–9.0 dKH, 400–450 ppm calcium, 1250–1400 ppm magnesium.
- Stability goal: alkalinity changes under 0.3 dKH per day.
- pH goal: 7.9–8.3 daily swing, with steady aeration.
For more baseline targets, see our reef tank water parameters guide. If your pH is unstable, review reef tank pH control tips.
How to test, track, and set a dosing plan
Start with good tests. Use a consistent method each time. Test alkalinity at the same hour daily for one week. Test calcium every two to three days that week.
Measure consumption before you dose. Example: Your tank is 8.5 dKH on Monday. It is 7.9 dKH on Tuesday. That is 0.6 dKH per day used. Dose to replace 0.6 dKH daily.
Make changes slowly. Limit alkalinity increases to 0.5–1.0 dKH per day. Limit calcium increases to 20–30 ppm per day. Fast jumps can stress acropora and clams.
Pick a system that fits your tank. Two-part dosing works well in most reefs. Kalkwasser helps when pH runs low. A calcium reactor suits high-demand SPS systems.
- Log every test in a notebook or app. Note time, dose, and livestock changes.
- Re-check alkalinity 30–60 minutes after dosing. Confirm the response.
- Calibrate salinity monthly. Aim for 35 ppt or 1.026 specific gravity.
If you are new to additives, read our two-part dosing guide. It covers mixing, pumps, and safe schedules.
Troubleshooting imbalance, precipitation, and “mystery” swings
A common issue is “alk won’t hold.” First, confirm test accuracy. Check expiration dates and technique. Then look for real demand from new frags, coralline, or a reactor change.
Another issue is white crust on heaters and pumps. That is calcium carbonate precipitation. It often happens when alkalinity is high and pH is high. It also happens when you add supplements too close together.
Separate dosing points and time. Dose alkalinity into a high-flow area. Wait 10–15 minutes before calcium. If using a doser, split daily dose into 6–12 small portions.
Watch for hidden causes. Low magnesium can trigger precipitation. High salinity can inflate calcium and alkalinity readings. A stuck ATO can swing salinity and stress corals fast.
- If alkalinity is high and calcium is low, stop alk dosing first. Raise calcium by 20 ppm per day.
- If calcium is high and alkalinity is low, stop calcium dosing. Raise alkalinity by 0.5 dKH per day.
- If both are falling, increase both doses by 10% and re-test in 48 hours.
Use water changes for resets. A 15–20% change can correct small drift. Match salinity and temperature first. Mix salt for 12–24 hours with heat and flow.
Conclusion
Alkalinity and calcium balance is about steady replacement, not big corrections. Test, log, and adjust in small steps. Your corals will reward you with better growth and color.
Keep alkalinity stable, keep calcium in range, and do not ignore magnesium. When in doubt, slow down and confirm your numbers before changing dose.
Sources: Randy Holmes-Farley, “Chemistry and the Aquarium” series; Craig Bingman, reef aquarium chemistry articles; Borneman, “Aquarium Corals” (calcification basics).
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