I've done that by mistake once with calcium and another time with alk.
I have a 80g with 30 g sump. I did a 50 g water change in one shot and all went well both times. One or two sps tips bleached but it came right back after a week or so.
Maybe you can dose some extra Cal into the tank and let it precipitate out the alk? I don't know how safe that is, just a thought. GL