Alk will hit its high point during the peak photo period. So if you test it’s always better to test at the exact same time. I like to test at the peak and then at the lowest decline. That way I have two reference points to know if something is going south.I just had mine rebuilt after 2 years of service. Some may go longer but I was getting inconsistent readings from test-to-test. Once the perk pump, lines, cuvet etc where changed it became consistent again.
you may find some surprises once you start testing at the same times each day especially in the early morning hours. Unless you’re testing at 2am-4am, I would have never seen my alk climb between those hours. Nothing dosed during those times either.