IMO %50 water change is a bit too large for a system thats already stressed as there really is no need to drop your alk that fast lol.
i would do a smaller water change and let the alk fall normally , 12dkh or even 16dkh is high but its not crash a tank high , as long as your calcium is relatively in range and no precipitation then there is no need for acid or large water changes and other things that are just going to give you more instability.
if you never experienced any rtn then i would go about your normal water change amount to help flush the nutrients and allow the alk to drop over the next few days.
if you are previously dosing then obviously there was consumption to begin with.
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