Reef Lab
Consumption calculator
Not "raise it from 7 to 8", but the question the tank actually asks every day: how much does it pull out, and what should the dosing pump therefore give?
Two readings
The mistake this fixes
Why a running pump changes the whole calculation.
If a pump is already dosing, the drop you measured is not your consumption. It is the part of your consumption the pump failed to cover. Everything else in the hobby's advice assumes you started from zero, and that assumption is what leaves people permanently under-dosed.
Say the tank uses 0.15 dKH a day and the pump supplies 0.10. Alkalinity still falls by 0.05 a day, so you measure a shortfall of 0.05 and calculate a top-up. Set the pump to that top-up alone and you have just cut your dosing by two thirds.
The correct new setting is the dose you were already giving plus the top-up. That single addition is why this calculator asks for what your pump is currently running.
Leave enough time between tests
A decent alkalinity kit scatters by around 0.1 dKH. Test on consecutive days and that scatter lands in the result undivided — you would be tuning the pump to noise. Over five days it is spread across five times the signal. Three days is the minimum, a week is better.
Worked through
The default values on this page, step by step.
drop = 8.4 − 7.8 = 0.6 dKH over 4 days
consumption = 0.6 / 4 = 0.15 dKH per day
top-up = 0.15 × (1 / 0.1) × (300 / 100) = 4.5 ml
new setting = 20 + 4.5 = 24.5 ml per day
300 litres net, Red Sea Foundation B, a pump already running 20 ml a day. The tank was short by
4.5 ml a day — not consuming 4.5 ml a day.
Consumption is a moving target
It follows the calcifying mass in the tank: it climbs as corals grow, steps up when you add livestock, and drops after fragging or a die-back. A setting that was right in spring will be wrong by autumn in a growing system. Re-measure every few weeks while things are changing.
When the value went up
Then you are adding more than the tank uses and the dose needs to come down. Same rules apply in reverse: move it gradually and re-measure rather than switching the pump off.
One more habit worth building: change one thing at a time. If you adjust the pump and do a water change in the same week, the next pair of readings cannot tell you which one moved the value.
Questions
How do I work out my tank's alkalinity consumption?
Test alkalinity, wait several days without changing anything, and test again with the same kit. The drop divided by the number of days is your daily consumption. Convert that into millilitres using your dosing product's rating and your net water volume, and you have the daily dose. If a pump was already running during those days, its dose has to be added on top — otherwise you are only measuring the shortfall it failed to cover.
Why does the dose I already run get added to the result?
Because with a pump running you are not measuring consumption, you are measuring the part of it the pump does not cover. If the tank uses 0.15 dKH a day and the pump supplies 0.10, the value still falls by 0.05 a day. Setting the pump to the top-up alone would make things worse. The new setting is the old dose plus the top-up, which is exactly what this calculator returns.
How many days should I leave between the two tests?
At least three, and a week is better. A good alkalinity kit scatters by roughly 0.1 dKH, and over a single day that scatter lands undivided in the result — you would be tuning your pump to test noise. Over five days the same error is spread across five times the signal. The calculator warns you when the interval is under three days.
My value went up instead of down. What does that mean?
You are adding more than the tank consumes, so the dose should come down rather than up. That happens after a water change with a high-alkalinity salt, when coral growth slows, or simply when the pump was set too generously. Reduce it gradually and re-measure — the same slow-change rules apply going down as going up.
Does consumption stay the same over time?
No, and that is the point of re-measuring. Consumption tracks the calcifying mass in the tank, so it climbs as corals grow, jumps after adding livestock, and falls when something dies back or after a heavy fragging session. A tank that has been stable for months will still drift. Re-check every few weeks in a growing system.
The other calculators
Same maths as the app, same free access.
Dosing calculator
Raise alkalinity, calcium or magnesium to a target, split over as many days as it takes.
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.
Let the logbook do this for you
Reef Lab already has your readings and their dates — it can work the consumption out from your own history and hand the result straight to the dosing pump. Free, works offline, no account to start.