Low alkalinity in a reef tank — diagnosis and fixes

DiagnosticTank has been growing well, alkalinity is dropping ~0.3 dKH per day or more. SPS adding visible growth (encrustation, polyp extension).

FixStart two-part dosing. Use the alkalinity dosing calculator to figure out daily dose. Begin at 50% of the calculated dose and increase weekly while testing alk + Ca.

DiagnosticRecent switch from Red Sea Coral Pro (11.5 dKH) to a low-alk salt (e.g. Tropic Marin Pro Reef at 7.5 dKH). Alkalinity dropped after the second or third water change.

FixEither go back to the higher-alk salt OR start two-part to make up the gap. There's no wrong answer; just pick a target dKH and stick to it.

DiagnosticYou have a reactor but alk is dropping anyway. Check effluent pH (should be 6.5–6.8) and effluent drip rate. Bubble counter has slowed.

FixCalibrate the pH probe (annual minimum). Check CO₂ regulator output. Increase effluent flow until daily uptake matches your tank's consumption.

DiagnosticMg <1,200 ppm. Alk and Ca both drop faster than dosing can replace.

FixRaise Mg to 1,300–1,400 ppm before continuing alk/Ca dosing. Without adequate Mg, Ca and CO3 precipitate together as calcium carbonate, removing both at the same time.

DiagnosticWere running kalk in top-off; switched to plain RODI. Alk dropped over 2–3 weeks.

FixResume kalk, OR start two-part to match the alk supply rate. Switching between methods requires bridging.