Reef tank pH swings — diagnosis and fixes

DiagnosticpH stays low all day, never crosses 8.0. Outdoor pH test of skimmer effluent reads near-normal seawater pH (8.2). Indoor CO₂ meter (Aranet4) reads >800 ppm.

FixRun a fresh-air line from outside to the skimmer intake. 1/4-inch airline through a window or vent. Single most effective pH intervention available in winter.

DiagnosticAlkalinity <7 dKH. pH is also low.

FixRaise alkalinity to 8–10 dKH gradually (≤1.4 dKH/day per Holmes-Farley). pH stabilizes within 24 hours of correcting alk.

DiagnosticpH swings >0.3 day to night. Photosynthesis spikes daytime pH, respiration drops it at night.

FixRun refugium on reverse photoperiod (refugium light on at night, off during day). This continuous photosynthesis cycle stabilizes 24-hour pH.

DiagnosticpH spike >8.5 immediately after kalk top-off triggers. Kalk dosing pumps run high volume in short bursts.

FixSlow drip via float-valve gravity feed, or spread doses across the day (≤25 mL/min for a 100-gallon tank). Never dump kalk all at once.

DiagnosticNo active skimmer, or skimmer rated for half the tank volume. CO₂ accumulates because gas exchange is limited.

FixRun a properly-sized skimmer (rated 2× display volume minimum) and ensure it pulls in fresh atmospheric air.