Reef Lab
Dosing calculator for reef tanks
How much to dose to hit your target for alkalinity, calcium or magnesium — and, just as important, across how many days. With KH the schedule matters as much as the amount.
Your tank
Why the dose gets split
The difference between a correction and a shock.
Alkalinity is not a number on a chart, it is the raw material corals build with. Move it quickly and they feel it immediately — receding tissue and burnt SPS tips in the days after a big correction are the classic signature. The safe ceiling this calculator works to is about 1.4 dKH per day, and going slower than that has never hurt anyone.
So a gap that is too wide for one day gets divided. Raising a 300 litre tank from 6.5 to 8.5 dKH with Red Sea Foundation B is 60 ml in total — but as 30 ml on each of two days, not 60 ml in one evening.
Corrections only go up
Every product here adds. Nothing takes alkalinity or calcium back out, so an overshoot can only be waited out or diluted away with water changes. That asymmetry is the reason to aim for the bottom of your target band rather than the top, and to re-test before topping up again.
Test after, not just before
A calculated dose is a prediction. Consumption keeps running while you correct, and test kits have their own scatter — roughly 0.1 dKH for a decent alkalinity kit. Measure again the next day and let the tank tell you whether the prediction held.
The arithmetic
No secret in it — the only two things that matter are the product's own rating and your real volume.
dose = Δvalue × (baseDose / effect) × (volume / referenceVolume)
Red Sea Foundation B is rated at 1 ml raising 0.1 dKH in 100 litres. For +1.0 dKH in 300 litres:
1.0 × (1 / 0.1) × (300 / 100) = 30 ml.
Never dose calcium and alkalinity together
Concentrated calcium and carbonate meeting each other precipitate as calcium carbonate. You see a white cloud, and afterwards both values are lower than when you started. Separate them by time of day, or at minimum by distance in the sump with strong flow between.
Check magnesium first when nothing holds
Magnesium is what keeps calcium and carbonate in solution at reef concentrations. Below roughly 1250 mg/l, alkalinity and calcium stop holding no matter how much you add — the additions precipitate instead of staying dissolved. If both keep falling, test magnesium before increasing either.
The product ratings here come from the manufacturers' own figures, the same catalogue the Reef Lab app uses. When a bucket or bottle states something different, the label wins.
Target values
What the calculator's suggestions are aiming at.
| Parameter | Optimal | Marginal | Critical |
|---|---|---|---|
| Alkalinity (KH) | 7.5–8.5 dKH | 7–9 | < 6 / > 10 |
| Calcium | 420–440 mg/l | 400–450 | < 380 / > 480 |
| Magnesium | 1300–1350 mg/l | 1250–1400 | < 1200 / > 1500 |
A pure SPS system runs tighter than this, a softie tank forgives more. All parameters, including nitrate and phosphate →
Questions
How fast can I raise alkalinity safely?
Around 1.4 dKH per day is the ceiling this calculator works to, and slower is better when corals are involved. Alkalinity drives calcification directly, so a fast swing is felt immediately — tissue recession and burnt SPS tips after a big correction are common. If the gap is larger than one day's allowance, the calculator splits the total across as many days as it takes and gives you the per-day amount.
Can I lower alkalinity or calcium with a dosing product?
No. Dosing products only add. To bring a value down you either wait for the tank to consume it, or you dilute with water changes using water mixed to a lower value. This is why overshooting is worse than undershooting: an overdose can only be corrected slowly, while a low value can be fixed the same day.
Should I dose calcium and alkalinity at the same time?
Never at the same point at the same moment. Concentrated calcium and carbonate solutions meeting each other precipitate as calcium carbonate — you get a cloud, and both values end up lower than before. Dose them at different times of day, or at least at opposite ends of the sump with strong flow between them.
Why is my magnesium relevant to alkalinity and calcium?
Magnesium is what stops calcium and carbonate from precipitating out of the water on their own. When magnesium sits too low, roughly under 1250 mg/l, alkalinity and calcium become difficult to hold no matter how much you dose — the additions simply precipitate. If both refuse to stay up, test magnesium before increasing anything else.
Should I use net or gross tank volume?
Net — the actual water in the system. Rock and sand displace a surprising amount, often 10 to 15 percent of the display, while a sump adds volume back. Using the number printed on the tank instead of the real water volume makes every dose too large, and the error is systematic rather than random, so it never averages out.
The other calculators
Same maths as the app, same free access.
Consumption calculator
Two readings, a few days apart — out comes the daily dose your pump should run.
Salinity calculator
Specific gravity to ppt with the temperature correction most converters skip.
Salt mix calculator
How much salt for your water change — and how to move a running tank's salinity safely.
A correction is only half the job
Reef Lab keeps the dosing history next to the chart, so you can see how the value answered your change — and adjust the pump instead of guessing. Free, works offline, no account needed to start.