| What It Is | Balanced calcium + alkalinity dosing |
| Part 1 | Calcium chloride |
| Part 2 | Alkalinity (carbonate/bicarbonate) |
| When You Need It | Corals consume more than water changes replace |
| Golden Rule | Dose to consumption; never mix parts |
Overview
Two-part dosing is the most popular way to keep calcium and alkalinity up as corals grow. It’s two separate solutions — one calcium, one alkalinity — added in balanced amounts to replace exactly what the corals consume. It’s precise, scalable from a nano to a big reef, and doesn’t require the equipment of a calcium reactor.
How It Works
As corals build skeleton they draw down calcium and alkalinity in a fixed ratio. Two-part replaces both in that same ratio, keeping the balance. You measure how much your tank consumes per day (the drop in alkalinity over 24 hours is the easiest gauge), then dose enough to offset it. As the coral load grows, the dose grows with it.
Step-by-Step
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- Test alkalinity two days running to learn your daily consumption (e.g. a 0.5 dKH/day drop).
- Start with the manufacturer’s dose for that consumption, split into smaller doses through the day if possible.
- Add Part 1 and Part 2 at different times or in different spots in high flow — never pour them together or back-to-back in the same spot, or they precipitate into a useless cloud.
- Re-test after a few days and fine-tune. Keep calcium and magnesium in range too.
- As corals grow, expect to raise the dose — recheck consumption periodically.
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Tips
A doser pump automates this and gives the steady, split dosing corals love. Keep magnesium in range or two-part won’t “hold.” For a small tank, water changes alone may cover demand until your corals grow in — two-part earns its keep once consumption outpaces changes.
Common Mistakes
Mixing the two parts: combine them and you get a snowstorm of precipitate — always separate in time and place. Dosing without testing: two-part is dosed to your consumption, not a label schedule. Big catch-up doses: raising alkalinity too fast burns corals — keep changes under ~1 dKH/day.