To keep alkalinity stable in a reef tank, measure how much your tank uses each day, then replace that amount on a steady schedule with water changes, two-part, kalkwasser, a calcium reactor, or an all-in-one supplement. Test at the same time of day with the same kit, and make any corrections slowly. A tank that holds 8.2 dKH every day is usually happier than one that bounces between 7.5 and 9.
What alkalinity is and why corals use it
Alkalinity is a measure of the water's buffering capacity, mostly carbonate and bicarbonate. In reef keeping it is usually written in dKH (degrees of carbonate hardness). Some kits report meq/L or ppm instead: 1 meq/L is about 2.8 dKH, and 1 dKH is about 17.9 ppm as calcium carbonate.
Stony corals, coralline algae, clams, and other calcifying animals pull carbonate and calcium out of the water to build their skeletons. That is why alkalinity drops in a growing reef, and why it drops faster as corals grow. Alkalinity also helps hold pH steady.
Reefability's default ideal range for alkalinity is 7.5 to 9.0 dKH. Treat that as a sensible starting point: every reef settles on its own number, and holding steady matters more than hitting an exact value. For the full list of parameters, see ideal water parameters for a reef tank.
How to measure daily consumption
Before you dose anything, find out how much alkalinity your tank actually uses. Consumption depends on your coral load, lighting, and growth rate, so another tank's dose rarely fits yours.
- Stop any alkalinity dosing for the test period, or keep it exactly constant so you can account for it.
- Test at the same time of day, ideally before the lights come on or at a fixed hour. Alkalinity often drifts through the photoperiod as corals calcify, so testing at random times adds noise.
- Test once a day for 3 to 5 days with the same kit.
- Subtract the last reading from the first and divide by the number of days.
For example, if you read 8.43 dKH on Monday at 9 a.m. and 7.89 dKH on Thursday at 9 a.m., the tank used 0.54 dKH over 3 days, or 0.18 dKH per day. That number is your starting dose target.
Tip: Repeat this check every month or two. As corals grow, consumption usually climbs, and a dose that was right in spring may fall short by autumn.
Once you know dKH per day, use your supplement's label to convert it into a volume, scaled to your net water volume (the water actually in the system, not the tank's rated size).
Dosing methods and their trade-offs
There is no single best method. It depends on how much your tank uses, your budget, and how much equipment you want to maintain.
| Method | Best suited to | Main advantages | Main trade-offs |
|---|---|---|---|
| Water changes | Light consumption, mostly soft corals | Simple, also replaces trace elements | Rarely enough on their own once stony corals grow |
| Two-part | Light to heavy consumption | Precise, easy to automate with a dosing pump | Needs regular testing and refilling; a pump fault can over- or under-dose |
| Kalkwasser | Light to moderate consumption | Raises pH, adds calcium and alkalinity in balance | Limited by how much water evaporates; can raise pH too far if overdosed |
| Calcium reactor | Heavy consumption, large stony coral systems | Supplies a lot of alkalinity and calcium with low running cost | Higher upfront cost, needs CO2 and tuning, can lower pH |
| All-in-one | Light to moderate consumption | One container, simple dosing | Less control over each element; may not match your tank's balance |
Water changes
A 10% weekly change only nudges alkalinity a small amount, so in a tank with real coral growth it tends to slide between changes. Water changes work well alongside another method.
Two-part
Two-part uses one solution for alkalinity (usually sodium bicarbonate or sodium carbonate) and one for calcium. Dose them separately, ideally a few minutes apart and in different spots in the sump, so they do not precipitate. Splitting the daily dose into many small doses through the day with a dosing pump gives the smoothest result. Carbonate based solutions raise pH more than bicarbonate based ones.
Kalkwasser
Kalkwasser (calcium hydroxide mixed in fresh water) adds calcium and alkalinity together and raises pH. It is usually dosed through the top-off, so the maximum you can add is limited by how much water evaporates. Many reefers dose it overnight, when pH naturally sits lower. Start small and watch pH.
Calcium reactor
A calcium reactor uses CO2 to dissolve a calcium carbonate media and drips the result back into the tank. It suits heavily stocked stony coral systems. Tuning takes patience, and the effluent can lower tank pH, so many reefers pair it with kalkwasser or extra aeration.
All-in-one supplements
All-in-one products put calcium, alkalinity, magnesium, and often trace elements in a single solution. Because the ratio is fixed, one parameter can drift if your tank's use does not match the blend, so test calcium and magnesium too.
How to raise or lower alkalinity slowly
A commonly cited guideline in the hobby is to change alkalinity by no more than about 1 dKH per day, and many reefers prefer less. Some keepers of sensitive stony corals aim for 0.5 dKH per day or slower.
To raise alkalinity:
- Calculate how far you are from where you want to be, for example from 6.91 to 8.2 dKH, a gap of 1.29 dKH.
- Split the correction over two or more days, for example 0.65 dKH per day.
- Add the correction on top of your normal daily dose, then retest the next morning before adding more.
To lower alkalinity, the gentlest approach is to reduce or pause dosing and let the corals use it up, testing daily. A water change with saltwater at a lower alkalinity also brings it down a little. Avoid adding acids to the tank to force it down.
The alkalinity, calcium, and magnesium relationship
Alkalinity, calcium, and magnesium are linked. Corals use calcium and alkalinity together, roughly 7 ppm of calcium for every 1 dKH of alkalinity, which is why balanced methods like two-part and kalkwasser supply them in that proportion.
Magnesium helps keep calcium and carbonate from combining and falling out of solution as calcium carbonate. If magnesium runs low, you may find it hard to hold calcium and alkalinity at all, and you may see white crust forming on heaters and pump impellers. Reefability's default ideal ranges are 390 to 450 ppm for calcium and 1200 to 1450 ppm for magnesium. Check magnesium first if alkalinity and calcium refuse to stay put.
Why test kits differ, and why to stick with one
Different test kits often disagree by a few tenths of a dKH, sometimes more. Reagent age, how you read the color change, and sample size all play a part.
That is why trends matter more than any single reading. Pick one kit and use it consistently so day-to-day changes reflect your tank rather than the kit. If you switch kits, run both side by side for a few tests so you know the offset. If a reading looks surprising, retest before you change anything.
Common causes of alkalinity swings
When alkalinity suddenly moves, check these first:
- Dosing pump issues: an empty container, a clogged or kinked line, or a pump schedule that changed.
- Coral growth: consumption rises steadily as colonies grow, so last season's dose may no longer be enough.
- New livestock: adding several stony corals at once raises demand.
- Precipitation: a large dose added in one spot, or low magnesium, can cause calcium carbonate to form and alkalinity to drop.
- Changes in nutrient control: carbon dosing and denitrification can raise alkalinity slightly, so changes in how you lower nitrate and phosphate may shift your numbers.
- Calcium reactor supply: an empty CO2 cylinder or exhausted media.
- A new salt mix: different brands and batches mix to different alkalinity levels.
Tracking it with Reefability
Logging every alkalinity test in Reefability shows each reading against your ideal range and charts it over time, so you can see your daily consumption at a glance. If you run a supported controller or dosing setup, the app can read it directly, and you can ask Reefer AI questions about your own tank's trend or set up a dosing plan with reminders.
Frequently asked questions
How often should I test alkalinity?
While you are working out your daily consumption or correcting a level, test once a day at the same time. Once your dose is dialed in and stable, two or three times a week is common, and more often in tanks with heavy stony coral growth.
Is 1 dKH a day too fast to raise alkalinity?
About 1 dKH per day is a commonly cited upper limit, not a target. If your corals are sensitive or you are unsure, split the correction over more days, for example 0.5 dKH per day.
What alkalinity should I keep my reef tank at?
Reefability's default ideal range is 7.5 to 9.0 dKH. Choose a level that suits your tank, then keep it steady rather than chasing a particular number.