Reef Tank Dosing Guide

As corals and coralline algae grow, they pull alkalinity, calcium and magnesium out of the water to build their skeletons. In a young or lightly stocked tank, regular water changes replace what is used. But once coral demand outpaces your water changes, you need to dose — add those elements back deliberately. This guide covers when dosing becomes necessary, what to dose, and the single habit that matters most: consistency.

When do you need to dose?

You need to start dosing when alkalinity (and usually calcium) drops measurably between tests despite regular water changes. The tell-tale sign is alkalinity falling, say, half a point or more across a few days. If your numbers hold steady on water changes alone, you do not need to dose yet — adding supplements to a tank that does not consume them just causes overshoot.

What to dose, and why

• Alkalinity is consumed fastest and is dosed most often. It buffers pH and supplies carbonate for skeletal growth. Most dosing routines are built around replacing daily alkalinity demand.
• Calcium is drawn down alongside alkalinity in a fixed ratio. A "2-part" system doses a calcium solution and an alkalinity solution separately so they do not precipitate in the bottle.
• Magnesium is consumed slowly but is essential: it keeps calcium and alkalinity dissolved together. If magnesium is low (below ~1250 ppm), you will fight constant alk/Ca precipitation no matter how much you dose.

Common methods include two-part solutions, kalkwasser (limewater, which adds Ca and alk together), and calcium reactors for high-demand systems. Beginners usually start with two-part because it is forgiving and easy to adjust.

Finding your daily demand

Test alkalinity at the same time on two consecutive days with no dosing. The drop is your daily consumption. Dose just enough to replace that amount, then retest to confirm the value holds flat. Raising a parameter is fine, but do it slowly — no more than about 1 dKH of alkalinity per day — to avoid stressing coral.

Consistency is the whole game

A reef does not care whether your alkalinity is 8 or 9 dKH; it cares that it does not bounce between them. Splitting a dose into smaller amounts spread through the day (a dosing pump shines here) keeps levels flatter than one big slug. Whatever your method, lock in a routine, change one thing at a time, and retest before adjusting again.