KH (Alkalinity): What It Means and Why It Matters in Your Aquarium
KH, or carbonate hardness, measures the concentration of carbonate and bicarbonate ions dissolved in aquarium water, and it’s what most hobbyists mean when they say “alkalinity.” It does two jobs at once: it buffers the water against sudden pH swings, and in a reef tank it supplies the raw carbonate corals pull out of the water to build their calcium carbonate skeletons. A KH test kit is one of the first things a new reef keeper is told to buy, and one of the last things an experienced one stops checking.
dKH, ppm, and How KH Is Measured
KH shows up on test kits in one of two units: degrees of carbonate hardness (dKH) or parts per million (ppm) of calcium carbonate equivalent. The conversion runs roughly 1 dKH to 17.9 ppm, so a reading of 8 dKH works out to about 143 ppm, and a reading of 300 ppm converts to roughly 16.8 dKH. Most dedicated reef test kits, including titration kits like Salifert’s, report directly in dKH, which is why that’s the unit most hobbyist discussion uses, even though ppm still shows up in some freshwater and pond product literature. A titration kit tends to be more precise than a colorimetric strip or liquid-drop test, which matters once KH is being tracked closely enough to catch a slow downward drift rather than just a crash.
Why KH Matters in a Reef Tank
Freshwater community tanks generally tolerate a wide KH range and rarely need active management. Reef tanks are a different story: most reef keepers target 8 to 12 dKH, and holding that number steady day to day matters more than hitting an exact figure. Corals consume carbonate continuously as they grow, so KH drifts downward in an established reef tank on its own and needs regular dosing or water changes to correct it. Calcium and magnesium are consumed alongside carbonate in the same reaction, which is why KH, calcium, and magnesium get called “the big three” and are tested and dosed as a set rather than one at a time. See Calcium & Magnesium for how those two interact with KH, and why a calcium or magnesium reading is easier to interpret once KH is known.
KH Crashes and Common Testing Mistakes
A KH crash, or a swing of more than about 1 to 2 dKH in a single day, is one of the more common causes of coral stress, tissue recession, and pH instability in an otherwise healthy tank. The usual trigger is dosing alkalinity supplements without testing first, or letting a two-part dosing system run unchecked for weeks while the tank’s coral load and carbonate demand keep growing. Because KH, calcium, and magnesium interact chemically, chasing a low KH reading with heavier dosing while magnesium is quietly low in the background limits how much carbonate the water can actually hold in solution, and the KH number won’t hold steady no matter how much is dosed. Testing KH weekly, alongside calcium and magnesium, is one of the highest-value habits in reef keeping, and logging results over time catches a slow drift long before it becomes a crash.
FAQ
What is KH?
KH is carbonate hardness, the measure of carbonate and bicarbonate ions dissolved in aquarium water. It buffers pH and, in a reef tank, supplies the carbonate corals use to build their skeletons.
What is dKH?
dKH stands for degrees of carbonate hardness, the unit most reef test kits report KH in. It measures the same carbonate and bicarbonate concentration as ppm, just on a different scale, and it’s the number most reef keepers quote when they talk about KH in a tank.
How do I convert KH from ppm to dKH?
Divide the ppm reading by roughly 17.9 to get dKH, or multiply dKH by 17.9 to get ppm. A tank reading 8 dKH is close to 143 ppm.
What’s a good KH level for a reef tank?
Most reef keepers target 8 to 12 dKH. Staying inside that range matters less than keeping the number stable day to day, since a fast swing is harder on corals than a slightly high or low but steady reading.
Why does KH keep dropping in my aquarium water?
Corals and coralline algae consume carbonate continuously as they grow, so KH falls over time in any established reef tank unless it’s replenished through dosing or water changes. A faster than usual drop often follows a recent coral addition or a growth spurt.
Shop KH & Alkalinity Test Kits
The Marine Aquarium Test Kits category covers dedicated KH and alkalinity testing, including the Salifert Marine Reef Test Kit: KH + Alkalinity. To correct a low reading, see Marine Aquarium Water Conditioner for buffer and dosing products, and how to run a Salifert titration test accurately for technique notes that apply to KH testing as well as calcium.