Maintaining stable calcium and alkalinity is essential for most reef aquaria. If you’re dosing kalkwasser (calcium hydroxide) or using two-part supplements or a calcium reactor, the goal is to keep calcium around 420–450 mg/L and total alkalinity roughly 8–12 dKH, while preserving pH health. Start with careful testing and small adjustments to avoid swings that stress corals and invertebrates.

Kalkwasser dosing basics
Kalkwasser is typically added through an automatic top-off (ATO) or a dosing pump to slowly raise calcium and alkalinity while buffering pH. Common starting points are around 0.5–2.0 mL per gallon of tank water per day via an ATO reservoir, adjusted after daily testing. If your alkalinity is low, you may increase slightly; if pH creeps above 8.4, slow down or pause dosing and verify CO2 and aeration in the tank.

Two-part dosing (calcium and alkalinity)
Two-part systems (often labeled Part A and Part B) separate calcium and alkalinity supplements. Typical starting targets for a 100–150 gallon system are to add enough calcium to reach ~420–450 mg/L and alkalinity to ~8–11 dKH. Begin with a modest daily addition (e.g., 10–20 mL of each part per day, then adjust based on daily test results). Use a reliable dosing pump and consistency in refill schedules. After installation, monitor calcium and alkalinity every 24–48 hours for the first week to dial in your rate.

Calcium reactors
Calcium reactors dissolve media (usually aragonite or similar) using CO2 to slowly release calcium and alkalinity. Start with conservative CO2 flow and a low effluent drip rate. A common method is to set reactor effluent to match about 5–10% of your tank volume per day as a starting target, then adjust based on test results. For a 100–150 gallon system, that might translate to roughly 0.5–1.5 gallons of reactor output per day, calibrated to maintain Ca ~420–450 mg/L and alk ~8–11 dKH. Carbon dioxide control is critical; too much CO2 can crash pH, while too little will slow dissolution.

Testing and monitoring
Rely on accurate tests for calcium, alkalinity, and magnesium (if you use magnesium dosing). Recheck after any adjustment, ideally daily for the first week. Track pH, salinity, and temperature, since these influence dosing effectiveness. Keep a log of dosing volumes, tank changes, and test results to spot trends and avoid overshooting targets.

Safety and compatibility tips
Avoid mixing products in the same container, maintain clean dosing lines, and ensure your return and circulation are adequate to distribute the dosed elements. If you notice rapid pH swings, sediment buildup, or coral tissue damage, pause dosing and reassess routes, volumes, and equipment. When in doubt, make small incremental changes and test frequently.

Next steps
Decide which method fits your tank size, confidence level, and maintenance routine. Whether using kalkwasser, two-part dosing, or a calcium reactor, a deliberate plan with regular testing and careful adjustments will help you stabilize calcium, alkalinity, and pH over time.