you cant really use a salt meant for human consumption in your tank for a few reasons
1 - stuff for human consumption usually has a few addatives. look at the contents, if theres anything other than sodium chloride in there its not pure salt
2 - the salt we use in aquariums is quite often synthetic and guraenteed with no polloutants harmful to our live stock and params inline with those found on the natural reef
3 - food sold as organic is usually very expensive, so id bet that per kilo there isnt much price difference
4 - our aquarium salts are usually a mixture of several different salts so it contains all trace minerals in the correct ratios
5 - normal salts are usually sky high with phosphates and nitrates once dissolved
6 - salt we eat is quite often taken from underground