[Innovator]

Do you have any corals in the tank? If so, how long has it been up and running with corals?

Edit: Well since you logged off I will go ahead and answer anyways. Alkalinity is much more "sensitive" than that of calcium and often fluctuates at a faster rate. Many of your calcifying organisms utilize calcium carbonate and bicarbonate for skeletal growth thus quickly depleting; however, this does not mean the system is imbalanced and rather you just need a higher level of supplementation (Calcium and Alkalinity) to achieve a higher balance. Note that water changes, top-offs, and nitrogen can alter the balance.The reason you see an overnight reduction in Alk is because seawater normally contains calcium in abundance to that of Alk so in order to make an impact a higher dosage is required. Raising your Mag will also help and altogether help stabilize a higher pH.