Question: After seeing some suspiciously low alkalinity readings on a new Red Sea pro test kit, I started thinking about some of the RHF articles I'd previously read on the topic. Specifically: That explanation of how standard alkalinity test kits work seems to be mirrored many places across the internet, but left me wondering... If the alkalinity test is simply measuring the amount of acid required to drop the PH down to 4.X, how can it be expected to produce reliable results, given that PH can vary significantly without simultaneously impacting alkalinity, such as with depressed PH due to high c02? I have to assume there's something more complicated happening than determining the difference between the current PH and 4.X. My understanding of chemistry is fairly basic, but I'm having a tough time reconciling how that would work; my first thought is that it probably has something to do with the alkalinity buffering the change somehow, but I have no idea if that's even close to right, and I'd like to have some grasp on the interactions at work here. Background: We built a new house this summer, and I was lucky enough to also pick up a Red Sea Reefer 750 (essentially 180 gallons) in August. I've been slowly getting things rolling, filling it with water and some appropriately cleaned dry BRS rock in September. I moved in a couple pieces of rock from an established tank, set up the lights on low and let the tank do its cycling thing in peace while I sat back and watched. Pod populations are healthy, the normal new-tank algae blooms have come and gone, the small handful of snails I put in to start munching away at the algae are all doing well, and things are generally looking pretty good with about 2.5-3 months of maturity. I recently moved over a couple of small sacrificial sps frags from my other tank out of curiosity, and they seem to be doing quite well so far. Their addition prompted me to start looking at Ca/Alk/Mag levels, rather than just watching PH/temp/conductivity/ORP on the Apex. Because this is a brand new house, I expect high c02 levels during the months where our windows are closed. My PH readings as per my Apex with new probes, calibrated in September have tracked my expectations pretty closely: troughs in the vicinity of 7.4 when the lights on the chaeto in the fuge are off, and highs of 7.65 or so with the windows closed and 7.8'ish with them open. I also see marked drops when the oven is used, and noted a very significant drop during the time lots of family was present on Thanksgiving, all aligning with low ph resulting from excess c02, but not dangerously so. My test results from Sunday evening using fresh red sea pro kits were: Ca: 450 Alk: 6.4 dkh Mg: 1520 and a PH of 7.76 This is with zero water changes or additions other than plain RODI topoff in 3 months - the water was mixed in a 50/50 ratio with IO and Red Sea Coral Pro. After looking at the BRS calculator, it seemed to indicate I need to add 160ml or so of the alkalinity portion of their 2 part to bring it up to 7.8. I decided to go slowly, and add 30ml twice a day... I did so for two days, testing nightly, but kept coming back to the essentially the same low alk value which prompted me to stop and evaluate before continuing. I have a terribly hard time imagining my mostly bare tank consuming that much alkalinity, so I'm now left waiting to try a second test kit to confirm the readings, and in the meantime wondering if my low PH could possibly be impacting my alk readings.