Net water, not tank rating. Use the Tank Volume calculator if you have not measured it.
Effluent drip rate to match your alkalinity demand, and what your current settings actually deliver.
Tune the effluent rate first and the CO₂ second. Raise CO₂ until effluent alkalinity reaches the target, then set the drip rate to match demand. Changing both at once makes the result impossible to read.
dKH/day = (30.0 − 8.5) dKH × 15.0 mL/min × 1440 min ÷ 1000 ÷ 284 L = 1.636 · needed rate = 9.17 mL/min
A calcium reactor takes tank water in, dissolves aragonite media in a low pH chamber, and returns water enriched in calcium and carbonate. What the tank gains per day is the enrichment times the volume passed through, spread over the system.
That gives dKH per day = (effluent dKH − tank dKH) × rate in mL/min × 1440 ÷ 1000 ÷ litres. Invert it for the rate you need: rate = demand × litres × 1000 ÷ (lift × 1440).
The lift matters as much as the rate. Effluent at 40 dKH into an 8 dKH tank does four times the work per millilitre that effluent at 16 dKH does. Running higher effluent alkalinity means a slower drip, which most reactors control more precisely.
Because the media is calcium carbonate, a reactor delivers calcium and alkalinity in exactly the ratio corals consume them, 7.16 ppm calcium per dKH. That is the reason to run one on a heavy SPS system rather than fighting a two part balance.
Tune one variable at a time. Set the CO₂ bubble rate until effluent alkalinity holds at target, wait a day, then set the effluent flow to match demand. Adjusting both together leaves you unable to tell which change did what.
Effluent pH is the other reading to watch, usually held between 6.5 and 6.7. Lower dissolves media faster but sends more undissolved CO₂ into the tank, which depresses tank pH. That is the standard reason a tank on a reactor runs low pH, and a kalkwasser top off or a CO₂ scrubber is the usual answer.
Media dissolves and packs down. Check the chamber every few months, because a channelled bed drops effluent alkalinity without anything else changing.
Measure effluent alkalinity from the effluent line itself, not from the reactor body or the sump. This is the most common tuning mistake.
Sources: Dieck, Calcium Reactor Calculator · Geo's Reef calcium reactor manual (PDF) · Bulk Reef Supply, calcium reactor setup · Melev's Reef, calcium reactor setup