About

In a reef aquarium, the stability of water parameters is more critical than the specific numbers themselves. Coral growth, particularly in calcifying species (SPS and LPS), directly consumes Carbonate Alkalinity and Calcium from the water column. By measuring the drop in alkalinity over a specific period (without dosing), a aquarist can determine the precise daily consumption rate. This data is the gold standard for calibrating dosing pumps or Calcium Reactors. Alternatively, tracking linear extension (physical size) helps monitor long-term health, though it is harder to quantify day-to-day.

Formulas

The daily consumption rate R is calculated by the difference between the starting value V_start and ending value V_end, divided by the time elapsed t in days.

R = V_start − V_endt

To convert Alkalinity from dKH to ppm (Carbonate Hardness):

ppm = dKH × 17.86

Balanced Calcium consumption can be estimated from Alkalinity drop using the ratio: For every 1 dKH drop, Calcium drops by approximately 7 ppm.

Reference Data

Parameter	Natural Seawater (NSW)	Recommended Reef Range	Daily Consumption (SPS Tank)
Alkalinity (dKH)	7.0 dKH	8.0 - 9.5 dKH	0.5 - 2.0 dKH/day
Calcium (Ca)	420 ppm	400 - 450 ppm	5 - 20 ppm/day
Magnesium (Mg)	1280 ppm	1300 - 1400 ppm	< 2 ppm/day
pH	8.1 - 8.3	8.0 - 8.4	Fluctuates (Day/Night)
Nitrate (NO3)	< 0.1 ppm	2 - 10 ppm	Variable
Phosphate (PO4)	< 0.02 ppm	0.03 - 0.1 ppm	Variable

Frequently Asked Questions

Chemically, they are consumed in a fixed ratio during calcification (formation of calcium carbonate). However, because the pool of Alkalinity in seawater is much smaller relative to Calcium, a small amount of consumption shows up as a large percentage drop in dKH, while the Calcium drop might be within the margin of error of your test kit.

Measure at the same time each day to account for pH swings. For a consumption test, turn off all dosing (Kalkwasser, 2-part, reactor) for 24 to 48 hours. Measure at Hour 0 and Hour 24.

If Calcium or Alkalinity levels are too high (or Magnesium too low), they can chemically bind and "snow" out of the water as solid calcium carbonate on heaters and pumps, rather than being used by corals. This causes a false high consumption reading.

As corals grow, their demand increases. Test weekly. If alkalinity is slowly trending down, increase the daily dose by 5-10% and re-test after a few days.