Two-part dosing is a simple way to keep alkalinity and calcium stable. It works well for many mixed reefs. This guide helps you dose safely and predictably.
Section 1: What two-part is and when to use it
Two-part usually means an alkalinity solution and a calcium solution. Many brands also suggest a magnesium supplement. The goal is stable coral building materials. Stability matters more than chasing perfect numbers.
Target ranges are straightforward for most reefs. Keep alkalinity at 7.5–9.0 dKH for mixed tanks. Keep calcium at 400–450 ppm. Keep magnesium at 1250–1400 ppm. Match your salt mix baseline first.
Two-part fits best when daily demand is moderate. Think small to medium SPS, LPS, and coralline growth. If you need more than 2 dKH per day, consider other methods. A calcium reactor or kalkwasser may scale better. Still, two-part can work with dosing pumps.
Do not dose blindly after a water change. Test 24 hours later. New saltwater can shift alkalinity fast. Review your reef tank water parameters before you start. It prevents common early mistakes.
- Test alkalinity at the same time each day for one week.
- Log results in dKH, not meq/L, for consistency.
- Confirm salinity at 35 ppt with a calibrated refractometer.
Section 2: Step-by-step dosing plan with real numbers
Start by finding your tank’s daily alkalinity consumption. Test alkalinity today, then again 24 hours later. Do not dose between tests. If alkalinity drops from 8.5 to 8.0 dKH, demand is 0.5 dKH per day. That number sets your daily dose.
Use your product’s calculator or label chart for exact mL. As a safety rule, raise alkalinity no more than 0.5 dKH per day. Raise calcium no more than 20 ppm per day. Big jumps can stress SPS and trigger tissue loss. Slow changes help keep pH steady.
Split doses across the day for smoother stability. Dose alkalinity and calcium at least 10–15 minutes apart. Dose into a high flow area, not near coral. Many hobbyists dose alkalinity at night for pH support. Dose calcium during the day to spread demand.
Example schedule for a 75-gallon mixed reef helps. Say demand is 0.5 dKH per day. You might dose 4 small alkalinity doses daily with a pump. Then match calcium based on testing, not assumptions. Re-test calcium twice weekly until it stabilizes.
- Week 1: Test alkalinity daily, calcium every 3 days, magnesium weekly.
- After stable: Test alkalinity 2–3 times weekly, calcium weekly.
- Adjust doses by 10% at a time, then re-check in 48 hours.
If you are new to dosing pumps, read our reef dosing pump setup walkthrough. It covers calibration and container safety. Calibration errors are a top cause of swings.
Section 3: Troubleshooting swings and avoiding common mistakes
Cloudy water after dosing often means precipitation. It happens when alkalinity hits calcium in a concentrated spot. Dose slower and into stronger flow. Keep alkalinity and calcium lines separated. Clean crust from pump tubing and outlets.
High alkalinity with low calcium can happen after aggressive alk correction. Stop alkalinity dosing for a day. Let coral consumption bring it down. Then correct calcium slowly. Re-check magnesium, since low magnesium can destabilize both.
Low alkalinity that will not rise often points to testing issues. Check expiration dates and technique. Rinse vials with tank water first. Verify salinity again, since low salinity lowers readings. Also check for hidden demand from new coralline growth.
Do not “equal dose” forever without testing. Demand changes as corals grow. It also changes after fragging or adding new SPS. Review your reef tank maintenance checklist monthly. It keeps your log and gear on track.
- Never mix Part A and Part B in the same container.
- Label jugs and lines to prevent accidental cross dosing.
- Keep solutions sealed to reduce contamination and evaporation.
Sources: Randy Holmes-Farley, “An Improved Do-it-Yourself Two-Part Calcium and Alkalinity Supplement System”; Julian Sprung, “The Reef Aquarium” (water chemistry sections); Seachem Reef Chemistry FAQ (alkalinity and calcium balance concepts).
By
