Two part dosing is the process of replenishing calcium carbonate that is used by corals and other inverts in building their skeletons and their shells, by adding separately, and in equal amounts, calcium chloride and sodium carbonate (or sodium bicarbonate).
It was invented to make your life easier by offering two separate solutions that could be dosed in equal amounts, thus adding calcium and alkalinity back into the water to help your corals thrive and grow.
Corals use the calcium from calcium chloride and the carbon from sodium carbonate to make calcium carbonate.
In a lightly stocked tank, a weekly water change will usually suffice in replenishing the calcium chloride and sodium carbonate. But as your collection of corals grow, they will consume more calcium and more alkalinity, probably outpacing the replenishment from the weekly water change, thus the need for two part dosing.
The reality is if you are only doing two part dosing and not doing water change very often, your corals aren’t thriving and growing as much as they could be.
Why is that? Because corals need more than just calcium and alkalinity to grow. They need other things such as magnesium, minor and trace elements.
Here are the items I used to set up my two-part dosing system.