freshly mixed at 1.028 (man it's hard to mix a small volume of salt water!) it tests at 10.92 dKH, so at 1.025 it would be lower, but still on the high side, probably in the 10s.
So I think I get what happened, and it was a stupid rookie mistake. The alk in the display was 6.2 when I did the water change. It was 6.35 after the water change, but that was after I let the whole tank homogenize for 10 minutes. For a good 40 seconds to a minute after the water change, the water coming out of the return nozzles would have been anywhere from 90% to 70% "new" water, so while the net change to the tank was small, the acutely localized swings would have been huge.
I don't know why I've never had this problem before unless H2Ocean mixes with a much lower alkalinity?
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