I have done 2 x 15% plus water changes in the last 4 days with no other chemical additions and my water parameters are the same as they were before except the Calcium is down to 440 but even after these changes my Alk is still 286 dkh. Bad salt bucket perhaps or will it take a lot of water changes?

Could be a nasty bucket of salt... what are you mixing it up to, 1.025? Have you measured your freshly mixed water?

see if you can borrow someones salifert kit so you can double check your readings.

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How is your Ph today?