Stable alkalinity is one of the most important keys to a healthy reef aquarium. While we often focus on calcium or lighting, it’s usually alkalinity swings that stress corals, slow growth, and trigger tissue loss. The good news: with a simple routine and the right tools, keeping alkalinity steady becomes almost automatic.

What Is Alkalinity and Why Does It Matter?

Alkalinity is a measure of your water’s ability to neutralize acids and keep pH stable. In reef tanks, it’s closely tied to coral skeleton growth and overall system stability. Most reef keepers aim for:

• 8.0–9.0 dKH for mixed reefs
• 7.5–8.5 dKH for SPS-dominant systems

More important than the exact number is consistency. A tank that holds 7.5 dKH every day is usually healthier than one bouncing between 7 and 9 dKH each week.

For a deeper foundation on water chemistry, check out our guide to reef tank water parameters and how they interact.

Testing and Tracking: The Foundation of Stability

You can’t stabilize what you don’t measure. Start by choosing a reliable alkalinity test kit or digital checker and build a simple schedule:

• New or growing reef: test daily for 1–2 weeks
• Established reef: test 2–3 times per week

Record each result in a notebook or app. After a week of consistent testing without dosing, you’ll see your tank’s natural daily alkalinity consumption.

Tip: Test at the same time each day. This removes daily pH swings as a variable and gives you cleaner data.

Dialing In Your Alkalinity Dosing

Once you know how much alkalinity your tank uses per day, you can match it with a dosing method. Common options include:

• Two-part dosing: Separate alkalinity and calcium solutions dosed throughout the day.
• Kalkwasser: Adds both calcium and alkalinity via your top-off water (best for moderate-demand tanks).
• Calcium reactor: Ideal for high-demand systems with dense coral growth.

Start by dosing slightly under your measured daily consumption, then adjust in small steps (0.2–0.3 dKH per day) until your readings stay flat. Use a dosing pump whenever possible to spread additions over 24 hours instead of a single large dose.

If you’re still planning your equipment, our article on reef dosing basics walks through setting up pumps and reservoirs for hands-off stability.

Common Causes of Alkalinity Swings

• Inconsistent water changes: New saltwater with very different dKH can cause sudden jumps.
• Changing salt brands: Different mixes often have very different alkalinity levels.
• Over-correction: Large one-time doses to “fix” a low reading can shock corals.
• Rapid coral growth: As corals grow, their demand climbs, and your old dosing schedule may fall behind.

Rule of thumb: Avoid changing alkalinity by more than 0.5 dKH per day unless you’re correcting a serious emergency.

Practical Tips for Long-Term Stability

• Mix and test new saltwater at least 24 hours before water changes.
• Match the alkalinity of your new water to your display tank as closely as possible.
• Re-test and re-calibrate dosing every few months as coral biomass increases.
• Pair alkalinity checks with your regular reef maintenance checklist so they become part of your routine.

Maintaining stable alkalinity isn’t about chasing numbers; it’s about creating a calm, predictable environment where corals can thrive. With consistent testing, measured dosing, and small, patient adjustments, your reef will reward you with stronger growth, better color, and far fewer surprises.