I would NOT use anything other than salt from the LFS
in your saltwater tank.
The main reason is first table salt has iodine in it. Iodine in excessive quantities is poison for your tank.
Second reason. While salt is a significant % of the makeup for something such as InstantOcean there are several other trace elements that are also in the mix. These trace elements are consitered essencial for saltwater life.
I'll try to find a link that shows what is in sea water and what is in Instant Ocean and other salt mix's.