ARM Effluent PH

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Using ARM in my single chamber, recirculating DIY reactor and keeping the effluent 6.7-6.8 When I first set up measured effluent at Ca 520ppm, Alk above 16KH (Salifert max) with ~40 drips/min at 20 bubbles/minute. Found tank was fairly steady with Ca 350-360ppm, alk ~10KH.

Little over a week ago thought would try for a higher Ca so added some Seachem Ca and I cranked up flow and bubbles to 60ml/min and 40 bubbles/min (effluent pH still 6.7-6.8). After a couple of days system seemed leveled to Ca380, alk 13KH, (tank pH 8.0-8.1) and I left on a trip. Just came home today and after being gone for a week, measured tank Ca 400 but alk 15.0KH (tank pH 8.0, effluent 6.7). I shut the CO2 off and guess I'll be doing a water change this weekend.