What Is Alkalinity (dKH) in a Reef Tank?

Alkalinity measures how much acid your water can absorb before its pH actually moves. In a reef tank specifically, it's carried by carbonate (CO₃²⁻) and bicarbonate (HCO₃⁻) ions, and those same ions are the raw material coral uses to build its skeleton. That's why alkalinity gets tested constantly while, say, chloride never does: it's both a stability buffer and a direct coral nutrient at once.

dKH, ppm, and meq/L are the same thing measured differently

Different test kits report alkalinity in different units, which trips up a lot of people comparing their reading to advice written for a different unit. Following Randy Holmes-Farley's reef chemistry reference (1 meq/L = 2.8 dKH = 50 ppm CaCO₃), the conversion works out to 1 dKH ≈ 17.86 ppm (as CaCO₃) ≈ 0.357 meq/L. If your kit reads in ppm and a guide talks in dKH, divide the ppm number by roughly 17.86 to get dKH.

The target range

Most reef tanks run well between 8-12 dKH (roughly 142-215 ppm) - Red Sea's own Reef Care Program guidance puts SPS-dominant tanks at 8.0 dKH, mixed reef at 11.5 dKH, and SPS frag tanks at 12.6 dKH, almost exactly this range. Where coral type fits inside that range is genuinely contested rather than settled. Some keepers run SPS-dominant tanks toward the higher end for faster growth, while others deliberately keep SPS lower for better coloration and stability. LPS and softie tanks are commonly kept toward the lower-to-mid end. Neither approach is objectively "correct."

Stability matters more than the exact number

Coral adapts to a stable 7.5 dKH about as well as it adapts to a stable 10 dKH. What it doesn't adapt well to is swinging between the two every few days, which is the part that actually trips up beginners chasing one specific target. If you're deciding between two "correct" numbers, pick the one you can hold steady, not the one that sounds more advanced.

These bands reflect common hobbyist practice within the 8-12 dKH range, not a settled standard. Whatever target you land on, holding it steady matters more than which row you picked.

What happens at the extremes

Alkalinity pushed too high on its own can outpace how fast coral tissue can lay down new skeleton, leaving thin tissue that burns at the tips under normal lighting, per ATI's alkalinity guide. That's a separate issue from calcium and alkalinity being high together, which Holmes-Farley's chemistry writeup ties to spontaneous calcium carbonate precipitation, a water-column and equipment problem more than a direct tissue-damage one. Alkalinity too low simply starves coral of the raw material it needs to build skeleton, which shows up as stalled growth long before it shows up as visible damage.