Very good measurements - a swing of 0.3 between full light and no light is rather normal. The small swing in the night is interesting - do you use a reversed refugium or algae scrubber?
Yeah i have a refugium.. the bottom spike is always around 6am-7am at the first alk dose which i thought was strange as it dosent spike with the other 2 doses during the day
I dose alk to maintain around 8dkh and about 10% of alk dose for calcium .... stays around 440.... so around 30 mil red sea alk and 3-4 mil red sea calcium dose 3 times a day.