I use food grade for all 3 since i started the hobby. No problems at all, salinity have been v.stable as i check salinity every 2weeks.
But chemically as kkk121 said, the sodium in kh and chloride from cal and mg can formed into sodium chloride which is salt , but is alotalotalot lesser than saltmix imo , big tank like yours shouldnt be an issue lol. Unless u no change water for very long time lah.. hahaha
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