I installed an Apex DOS 13 months ago to dose Ca and Alk. It never occurred to me until earlier this week that maybe I should check the calibration. So I went through the calibrated routine and the DOS pumped 38 ml on one side and 36 ml on the other. Doesn't seem like a big variance but those are 5% and 10% variances. So I I added to my husbandry checklist to recalibrate the DOS every 6 months. How often do you recalibrate your DOS?