I've held back on the vinegar for now: I measured ph when I first realized what had happened---[my upstairs tank went opaque]. It was 9.4. In the time it took me to run downstairs, find a bucket, find the vinegar, measure out a gallon and try some vinegar, the tank water had dropped on its own to 8.9, so it's coming down fast.
WHat happened...I was installing a timer on my stirrer, and got a shiny new extension---which it didn't fit; had to go to plan b. Meanwhile looked at that tatty plug on the topoff pump, decided it would fit better on the other extension, congratulated myself it went right in- [stupid me! the other extension is the one connected to the float switch! I'd just told the topoff to start running and keep going. So it blitzed salinity as well as dumped my whole kalk reactor kalkwasser supply into the tank....
And what are my corals doing as the dust settles? Open wide and sucking it all in as if, hitherto, I'd starved them.
I really, really may get out of this with a tank still functioning, never mind the reefer's own stupidity. Doh---let's just move this plug over so there's not a gap---doh!
THank you all so much for good advice: more water's ro/di'ing, I got at least 7 g saltwater mixing [stole water from the topoff barrel] and I have both vinegar and buffer standing by. Plus a turkey baster. WHen you do something this abysmally stupid, it's a real comfort to be able to go onto RC and know you're hearing advice from people with their heads on straight!