I did the aeration method for a few days using a sump pump and by turning the return nozzles up, just to see what would happen. The alk dropped from 220 to 170 but then hung there, using lots of acid in the meanwhile to keep the pH down. It's a plaster pool and the fill water is 235ppm alk, so it's a losing battle. There's no evidence of scaling so I'm going to leave it. From keeping aquariums I've learned, don't fight the water if you don't have to.

It's a shame I had to turn the nozzles back down, we sorta liked the water effects.