With the KHD I was testing 24 times a day and I got my alkalinity stable within 0.1 DKH and that was the best I could do, yes photosynthesis definitely effects things, for some reason I noticed that my alkalinity consumption actually increases when my lights shut off vs my lights onI would add that Photosynthesis and pH both play a roll in the results here, among other factors, which are difficult to control. I only test ALK every four hours as evident in the graph. I am not sure that 100% stability is possible but that is a good goal.