Keeping calcium and alkalinity stable is one of the biggest keys to thriving corals. Two part dosing is a simple, proven way to maintain these levels without constant manual adjustments. In this guide, we’ll walk through what two part dosing is, how to set it up, and how to dial it in for a healthy reef.
What Is Two Part Dosing?
Two part dosing is a method of adding balanced supplements to replace what corals and coralline algae use up every day. The “two parts” are:
- Part 1 – Alkalinity: Usually a carbonate/bicarbonate solution that stabilizes pH and supports skeleton growth.
- Part 2 – Calcium: A calcium chloride solution that keeps calcium levels in the ideal range for stony corals and clams.
Many hobbyists also add a magnesium component, since proper magnesium helps keep calcium and alkalinity from precipitating out of solution.
If you’re still planning your setup, read our overview on reef tank setup basics to make sure your foundation is solid before you start dosing.
How to Start Two Part Dosing
1. Test Your Baseline
Before dosing anything, test your water for:
- Calcium (ppm)
- Alkalinity (dKH or meq/L)
- Magnesium (ppm)
Compare your readings to your target range. Many mixed reef keepers aim for:
- Calcium: 400–450 ppm
- Alkalinity: 7–9 dKH
- Magnesium: 1250–1400 ppm
2. Correct Levels First
Use a reef calculator and your chosen two part product to bring calcium and alkalinity into range before you establish a daily dose. This is a one-time adjustment, not your long-term schedule.
3. Find Your Tank’s Daily Consumption
This is where the magic happens. You want to match what the tank uses each day.
- Get your levels into the target range.
- Stop dosing for 24 hours.
- Test again at the same time the next day.
- Note how much calcium and alkalinity dropped.
Tip: Repeat this over 2–3 days and average the results. This gives you a more accurate consumption rate.
Use your supplement instructions or a dosing calculator to convert that daily drop into milliliters of each part. That number becomes your starting daily dose.
Dialing In Your Two Part Dosing Schedule
Manual vs. Automatic Dosing
You can add two part manually with measuring cups or syringes, or use a dosing pump for precision. If you’re not sure which path is right for you, check out our comparison of automatic vs. manual dosing options.
- Manual: Best for smaller tanks or beginners; dose once or twice a day.
- Dosing pump: Ideal for stability; spread dosing into many small additions over 24 hours.
Best Practices for Stable Parameters
- Never mix the two parts together. Dose them separately, a few minutes apart, in a high-flow area.
- Test weekly at first. Adjust your dose if levels drift off target.
- Increase slowly. Change your daily dose by no more than 10–20% at a time.
- Watch your corals. Polyp extension, color, and growth tips are great indicators of success.
- Re-check magnesium monthly. Low magnesium often explains why calcium and alkalinity won’t stay put.
As your corals grow, their demand rises. Expect to tweak your dosing every few months. For more on keeping chemistry in check, see our reef water chemistry guide.
When Things Go Wrong
If you overshoot and levels get too high, stop dosing that component and let the tank naturally use it up. Avoid large, sudden corrections. Stability is more important than chasing a perfect number.
Two part dosing may sound technical, but once you’ve measured your tank’s daily use, it becomes a simple routine. With consistent testing, small adjustments, and a bit of patience, you’ll keep calcium and alkalinity rock-solid—and your reef will reward you with steady growth and vibrant color.