Marine Fish Tank Additives

why is salt Worth 50 cents a kilo when added to a marine tank, worth $15 per kilo?

Salt is evaporated from sea ponds, it is safe enough for humans to eat yet not safe enough for a fish tank. We are advised that marine fish tank salt is frree from any additives , yet so is non idisoed salted and salt free fom caking agents, is this the world’s biggest rip off?
I am in australia so we dont have mined salt, or beat bogs , just lots of sun and evaporated salt from the sea

the salts i use here in the US are synthetic and are manufactured but they work fine i mean if you think about it if you just let sea water evaporate and take the salt left over and added back too fresh water it would actually be better to use cuz it would have all the essential minerals that it had when it was in the ocean so if you could just calibrate the gravity and pH you should be fine.

Marine saltwater fishtank using tap water. Don’t believe the hype.