July 18, 2026

How to calculate daily alkalinity consumption in a reef tank

If your corals are growing, your alkalinity is falling. That is not a problem to fix, it is the tank telling you it is working. Calcification pulls carbonate out of the water to build skeleton, and every day your corals take a little more.

The question is how much. Not "some", not "a lot lately". A number, in dKH per day, that you can turn into a dose. Once you have it, dosing stops being guesswork and alkalinity stops sawing up and down between water changes.

Here is how to measure it properly.

What consumption actually means

Daily alkalinity consumption is how far your alkalinity would fall in 24 hours if you added nothing. It is always expressed in dKH per day.

That is the whole idea. If your tank drops 0.6 dKH every day, you need to add 0.6 dKH every day to hold steady. Dose less and you drift down. Dose more and you drift up.

For reference, here is roughly where tanks land:

Use that to sanity check your result, not to skip the measurement. Two tanks the same size can differ by a factor of ten.

The method

Three rules. All three matter.

1. Hold your dosing constant. The cleanest measurement is with dosing off entirely, because then the drop you see is pure consumption. If your tank consumes enough that switching off would crash it, leave the dose exactly as it is and do not touch it for the whole test. You will be measuring net drift instead of raw consumption, which still tells you exactly how much to adjust.

2. Test at the same time of day, every day. This is the rule people skip, and it wrecks the result.

Coral calcification is light driven. Under the lights, photosynthesis runs, the corals lay down skeleton faster, and alkalinity falls faster. At night the rate slows down. Consumption is not spread evenly across 24 hours, so a reading taken at 8am and the next one at 8pm are not a day apart in any meaningful sense. Test at the same clock time and every interval is a genuine, comparable 24 hours. Pick a time you can actually hit, an hour or so before lights out works well, and stick to it.

Sample from the same place too. Mid water column, in good flow, away from where your doser drips in.

3. Run it for at least four days. Not two. The next section explains why, and it is the single biggest reason people end up with a wrong number.

Also: stop the test early if alkalinity falls below about 7 dKH. Getting a clean number is not worth burning tissue to do it.

If you would rather not do the arithmetic by hand, the alkalinity consumption calculator on this site takes your dated readings and works out the daily figure for you, including a warning when your readings vary by more than your test kit can honestly resolve.

The arithmetic, worked through

Say you test at 8pm for five consecutive days with dosing off:

The daily drops are 0.3, 0.4, 0.3 and 0.3. Add them up and you get 1.3 dKH, then divide by 4:

1.3 ÷ 4 = 0.325 dKH per day. Call it 0.33.

The shortcut is the same thing: first reading minus last reading, divided by the number of days elapsed. That is 8.9 minus 7.6 = 1.3, over 4 days, equals 0.325 dKH/day.

Note that you divide by 4, not 5. You have five readings but only four 24 hour intervals between them. Dividing by the number of test results is the most common mistake in this calculation and it will understate your consumption by 20 to 25 percent.

Turning that into millilitres

Now convert. You need two things: your tank's real water volume and your supplement's dose rate.

Real volume is not the number on the box. Take display plus sump, then subtract roughly 10 to 15 percent for rock and sand displacement. A "75 gallon" system with a sump might hold 85 gallons of actual water, or 70. Guess badly here and every dose you calculate is wrong by the same percentage.

Dose rate is on the bottle, usually as something like "10 ml raises 1 dKH in 25 gallons".

Worked through, for a system holding 50 gallons of water:

• 10 ml per 25 gallons means 1 dKH in this tank needs 20 ml
• Consumption is 0.33 dKH/day
• 0.33 × 20 ml = 6.6 ml per day, call it 7 ml

That is your maintenance dose. Split it into four doses of about 1.7 ml.

Again, the calculator will do the consumption half of this for you if you would rather just enter your readings and get the number.

Your alkalinity is now sitting at 7.6 after the test, and you want it back at 8.8. Do not add that in one go. Add a small correction on top of the maintenance dose and spread it over three days, keeping the daily rise under 0.5 dKH. Then drop back to maintenance and let it sit.

Your test kit is the limiting factor

This is why four days matters.

Alkalinity kits vary enormously in resolution:

• Hanna checker (HI772): reads in ppm, 1 ppm resolution, about 0.056 dKH per step, with roughly 5 percent accuracy on the reading
• Red Sea Pro titrator: about 0.05 dKH per drop
• Salifert: about 0.1 dKH per drop
• API and other cheap drop kits: roughly 1 dKH per drop

Now put that next to a real consumption number. If your tank uses 0.3 dKH per day and your kit resolves 1 dKH per drop, a single day's drop is invisible. It rounds to zero, or it rounds to a full drop and you conclude your tank eats 1 dKH a day, which is three times the truth.

Over five days that same tank drops 1.2 to 1.3 dKH. That is more than one full drop on even the coarsest kit, so it is measurable. The multi day window is not about being thorough, it is about making the total change large enough that your kit's error stops dominating the result.

The same logic applies to good kits. A 0.1 dKH kit still carries operator error: how you read the colour shift, how consistently you hold the syringe, room lighting, reagent age. Over one day that noise can be as big as the signal. Over five days it averages out.

A few practical habits that cut error:

• Same kit, same reagent bottle, same person, for the whole test
• Read the endpoint in consistent light, not next to a window at different times of day
• If one reading breaks the pattern hard (say 8.9, 8.6, 9.1, 7.9), it is almost certainly a bad test. Retest rather than averaging it in.

Consumption is not a fixed number

The number you just calculated is true for this month. It will not stay true.

It goes up when:

• Corals grow. More skeleton laid down per day means more demand, and this creeps up steadily rather than jumping. A tank at 0.4 dKH/day in spring can be at 0.9 by autumn with nothing added.
• You add SPS. Acropora and friends are the heavy consumers. Softies barely register.
• Coralline algae takes off. Genuinely underestimated. A tank with purple covering every surface is consuming a real share of the total.

It goes down when:

• You frag and sell. You just removed biomass, and demand drops with it.
• Corals are unhappy. This one matters more than the rest, and it gets its own section below.

Recheck every four to six weeks, and any time you notice alkalinity drifting off target while your dose has not changed. That drift is the signal that consumption moved.

One cross check worth knowing: calcification consumes calcium and alkalinity together in a fixed ratio. Every 1 dKH of alkalinity consumed corresponds to about 7.1 ppm of calcium. So a tank eating 1 dKH per day should be dropping roughly 7 ppm of calcium per day. You will not see that in a single day (calcium kits are not precise enough to resolve 7 ppm), but over a week it is about 50 ppm, which is clearly visible. If alkalinity is falling fast and calcium is not moving at all, corals are probably not the thing consuming it.

Why you split the dose

Once you have your daily millilitres, resist dumping it in at once.

The safe rate of change for alkalinity is about 0.5 dKH per 24 hours, and plenty of experienced keepers hold corrections to 0.3. That is the ceiling for deliberate changes. Now consider a tank consuming 1.5 dKH per day: dosing all of that in one shot moves alkalinity 1.5 dKH in a couple of minutes, three times past the safe rate, every single day. The tank spikes at dose time and slides down for the next 23 hours. Corals sit in a sawtooth.

Split it and the swing collapses. Four doses a day means each one moves alkalinity by a quarter as much. A doser running hourly makes the change effectively invisible, which is the entire reason dosers are worth the money once consumption climbs past about 1 dKH per day.

Two practical points:

• Dose into high flow, so it disperses before it contacts anything. Not into a skimmer intake.
• Separate alkalinity and calcium doses by at least 30 minutes and dose them in different locations. Together in concentrated form they precipitate, and you lose both.

The signal almost everyone misses

Here is the part worth remembering after everything else fades.

Consumption is a biological output. It is a direct measure of how hard your corals are working. So when consumption drops suddenly, that is not good news and it is not a reason to quietly reduce your dose.

What it looks like in practice: your dose has held alkalinity at 8.5 for months. Nothing changed. Then alkalinity starts climbing on that same unchanged dose. 8.5, then 8.8, then 9.2.

Your corals stopped using it.

That happens before tissue recession, before browning, before polyps stay closed. Days before, sometimes a week. Common causes are nutrients bottoming out (nitrate or phosphate at zero), a temperature or salinity swing, an alkalinity swing of your own making, a pest, a contaminant, or something dying in a rock you cannot see. If it started right after a water change, that narrows things down considerably and corals not opening after a water change covers the likely causes.

The wrong response is to cut the dose so the number goes back to target. That hides the warning and changes nothing underneath. Cut the dose enough to stop alkalinity climbing past your safe range, then go find out what changed.

This is also the strongest argument for logging every test rather than testing and forgetting. A single reading of 9.2 dKH means almost nothing. A chart showing 8.5 flat for two months and then a steady climb on a fixed dose tells you exactly when something went wrong, which makes it far easier to work out what you changed that week.

The short version

1. Hold your dose constant or turn it off
2. Test at the same time of day for four to five consecutive days
3. First reading minus last reading, divided by the number of days elapsed, not the number of tests
4. Multiply by the millilitres that raise your tank 1 dKH
5. Split the result across the day
6. Recheck every four to six weeks, and treat a falling consumption as a warning

Tracking this by hand works, and the arithmetic above is all there is to it. Recif is the reef companion app we are building to handle it for you: log each test and it calculates consumption across your trailing readings, charts alkalinity against your target band, and flags the drift before you notice it. It is still in development and not yet released, but you can see where it is headed on our apps page.