Alkalinity and calcium drive coral growth in reef tanks. When they drift, corals stall and algae often wins. This guide explains targets, testing, and dosing with simple steps.
What alkalinity and calcium do in a reef tank
Alkalinity measures your water’s buffering and carbonate supply. Corals use carbonate to build skeleton. Stable alkalinity also helps keep pH from swinging. In reefs, stability matters more than chasing a perfect number.
Calcium is the other building block for stony corals and coralline algae. If calcium runs low, growth slows fast. If calcium runs very high, precipitation can strip alkalinity. That can leave you with cloudy water and low dKH.
Magnesium sits in the background and reduces precipitation. It helps keep calcium and alkalinity available. A common target is 1250–1400 ppm. Low magnesium often causes “can’t keep alk or calcium up” complaints.
Use these starting targets for mixed reefs. Alkalinity: 7.5–9.0 dKH. Calcium: 400–450 ppm. Keep salinity stable at 35 ppt. Check your salt mix baseline before you dose anything.
- reef tank water parameters overview for safe ranges
- Alkalinity change limit: 0.5 dKH per day
- Calcium change limit: 20 ppm per day
Testing routine and reading your results
Test alkalinity often, because it moves fastest. In new SPS tanks, test daily for one week. In stable mixed reefs, test 2–3 times per week. Test calcium weekly until you learn your tank’s demand.
Always test at the same time of day. Many tanks show a small daily pH cycle. That can change how you interpret results. Record results in a log with date, time, and dosing amount.
Watch for red flags in the trend. A steady drop in dKH means demand exceeds dosing. A steady rise means you are overdosing. Big swings often come from missed doses or inconsistent top-off salinity.
Example: Your alkalinity drops from 8.5 to 7.7 dKH in 48 hours. That is 0.8 dKH over two days. Your daily demand is 0.4 dKH. Set dosing to replace about 0.4 dKH each day, then re-test.
- Rinse test vials with tank water before testing
- Use a white background for color endpoints
- Re-check any “surprise” result with a second test
Dosing methods, adjustments, and common mistakes
Two-part dosing is the most common method. One bottle raises alkalinity, the other raises calcium. Dose in separate high-flow areas. Wait at least 10 minutes between parts to reduce precipitation.
Kalkwasser is another option, usually added through top-off. It raises alkalinity and calcium together. It also boosts pH, which helps in low-pH homes. Start weak, like 1 teaspoon per gallon of RO/DI, then increase slowly.
Calcium reactors work well for high-demand SPS systems. They add balanced alkalinity and calcium by dissolving media. They require stable CO2 and effluent control. They also need frequent alkalinity checks during tuning.
Common mistakes are easy to fix. Do not “correct” both alkalinity and calcium in one big jump. Fix alkalinity first if it is low. Then adjust calcium the next day. If you see white dust on pumps, lower dosing and check magnesium.
- If dKH is low: raise 0.3–0.5 dKH, then re-test in 6–12 hours
- If calcium is low: raise 10–20 ppm, then re-test next day
- If both drift: check salinity with a calibrated refractometer
When you feel stuck, simplify the system. Verify test kit expiration and technique. Mix a new batch of saltwater and test it. Compare to your tank. Also review your reef dosing guide settings and pump calibration.
For long-term success, aim for repeatable habits. Dose smaller amounts more often. Automate if possible. Pair this with steady nutrients and flow. See SPS coral care basics if you keep demanding corals.
Sources: Randy Holmes-Farley, “An Improved Do-it-Yourself Two-Part Calcium and Alkalinity Supplement System” (Reefkeeping Magazine); Randy Holmes-Farley, “Alkalinity in Reef Aquaria” (Advanced Aquarist); Julian Sprung, The Reef Aquarium (reef chemistry chapters).
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