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Good point--I think in my case, 3dKH turned to 4, then 5, then 8 (over a span of a few days), with each rapid fall happening over a period of an hour or two, not a whole day, then a cloud of precipitate on the water's surface ... all just points to what several of the others said. (One interesting thing I saw, if I dosed the carbonate, the exact time it took to complete the whole dose is exactly the time it took for alk to drop back down to where it was, or lower. So if it took me 2 hours to complete the dose, 2 hours later, it would be back down where it was--which led me to the assumption that it was all precipitating out exactly as quickly as I was putting it in.) After swapping, alk has remained steady, consumption is just under 1dKH a day, and pH stays locked on 8.1, even at night.
Jonify - One side note about your observation. It's certainly possible that what you were seeing on the water's surface is precipitated calcium carbonate from a high-pH addition of sodium carbonate. And once calcium carbonate forms, it will not redissolve in seawater to any appreciable extent that a reef keeper would care about. The note here is that the high pH of an alkalinity addition also causes a very temporary precipitation of magnesium in the form of magnesium hydroxide. This is the quickly-clearing "cloud" that reefkeepers that dose by hand have observed. Since magnesium hydroxide is not stable as a solid in seawater unless the pH is about 9.5, you can observe the specifics of the precipitation to determine whether you're forming magnesium hydroxide, calcium carbonate, or both - the Mg(OH)2 will dissipate almost instantly, while a precipitate that lasts more than a few seconds is calcium carbonate.