A) they are not the same minerals
B) is kosher salt the one that's pure NaCl?
C) there's other kinds of salt in marine salt than are in food salts, potasium salts, magnesium, etc..
if you want to not have a skimmer you'd want like 100 gallon tank with like 120 pounds of LR and do weekly 15% or so WC's... a skimmer is very advised for larger messy fish like them...