What Should Alkalinity Be in a Reef Tank?

What alkalinity actually measures

Alkalinity is the water’s capacity to resist a fall in pH. In seawater it’s mostly bicarbonate and carbonate. Corals and coralline algae use those to build their skeletons, which is why alkalinity drops as a reef grows.

It matters for two reasons. Low alkalinity slows skeleton growth. And because it buffers pH, it also helps keep pH from swinging too low overnight.

Choosing your number

Randy Holmes-Farley suggests 7–11 dKH (2.5–4 meq/L) for reef aquariums. Hanna Instruments gives 8–12 dKH. These ranges overlap, and most tanks do well somewhere in the middle.

If you’re starting out, a target around 8–9 dKH gives you room either side. Some salt brands mix up at a particular alkalinity, and matching that number makes water changes gentler. Check your salt’s label or the maker’s guidance. If you keep mostly soft corals, consumption is often low enough that your salt alone holds it steady.

1 meq/L = 2.8 dKH = 50 ppm as CaCO₃. Use the alkalinity converter for any other reading.

Why stable beats perfect

Corals adjust to the water they’re in. A tank that sits at 8 dKH every day is usually in a better place than one that bounces between 7 and 10 as you chase a number. Large, fast changes are the thing to avoid.

When you do need to change alkalinity, move it slowly. Bulk Reef Supply suggests raising it by no more than 1.4 dKH a day. Our dosing calculator splits a correction over several days for you.

Too high, too low

Below about 7 dKH, coral growth slows and pH is more likely to dip. If it keeps falling, see why alkalinity drops.

Above about 11–12 dKH, calcium carbonate can start to come out of solution. It forms a crust on heaters and pumps and takes calcium with it. Hanna warns about this kind of precipitation at high alkalinity. If you’re high, stop dosing and let the tank use it up slowly, rather than doing a big water change to knock it down.

Testing alkalinity properly

Test at the same time of day, because alkalinity and pH shift a little between day and night. Use the same kit, check its expiry date and follow its instructions exactly. If a reading surprises you, test again before acting on it.

Test weekly in a steady tank, and every day or two while you’re setting up or changing a dose. Logging each result shows how many dKH your tank uses per day, which is the number you need to set a dose.

Is 12 dKH too high for a reef tank?

It’s above most published ranges. Many tanks cope, but precipitation becomes more likely. If it’s steady and your corals are fine, bring it down slowly by reducing dosing, not with a big water change.

Is 6 dKH too low?

It’s below the usual ranges. Raise it gradually, by no more than about 1 dKH a day, and find out why it fell.

What’s the difference between KH and dKH?

They’re the same thing. KH is carbonate hardness and dKH means degrees of carbonate hardness. Both describe alkalinity on the same scale.