To throw another wrench into the issue - You also have to 'learn' about the type of alkalinity being measured. I was just schooled on this the past week. Titration methods meausre 'Total Alkalinity" - which is comprised of Carbonate Alkalinity and Borate Alkalinity. Corals due not consume both of these types of alkalinity at the same rate. The Borate Alkalinity is a 'value' that is different for different aquarium system. So during the day what's really going up and down is the Carbonate at a much greater rate than Borate. But most of the testing methods only measure Total Alk. Some aquariums the Borate Alkalinity can be under 5% of the total. And in other systems it could be 40%+. If you have one of the aquariums where the Borate is 40%. Then you could see stable Alk readings, when in fact your true Carbonate is changing more than you would like.
And you could also think you are on target with a dkh of "8.5" - But in fact the carbonate alkalinity of your system could be way below what you really want to target. And your non-connected frag tank with a dkh of 8.5 has a much lower Borate Alkalinity level which means the Carbonate Alkalinity is much higher ie closer to the 8.5 you want.
Doesn't that make things more confusing
Dave B