There is NO BEST salt. Most are quite similar. Some have higher CA
,, and lower Alk
, some have the reverse. Pretty much any of the quality name brands should be good.
I did have a problem with Kent's a few years ago that was verified by them (sent them samples from two buckets).
The search for the BEST salt mix is right up with the search for any other mythical object. It can also start a holy war with everyone claiming their salt brand is the best.
However, doing regular water changes is the best thing you can do for your livestock. Pick a brand and use it. If it lacks something you can add or change brands later.