Scott is correct, most shorter (under several 100 word) posts should be diaries. So next time, for a "shorty" please make it a diary. (Anyway they often get responded to more quickly and they go immediately to the top of the diary list and the "everything" page without having to wait for a vote.)

You are certainly right about the dangers of table salt. Even the ones which use flour to keep the salt flowing, can then pollute an aquarium. There are a lot of commercial marine salts. Interestingly their recipes are often a little different one from another. They do contain trace elements, but while those are important in a marine aquarium, they may not be needed in a brine shrimp hatcher. Probably most any one could be used in a freshwater aquarium to medicate.

I would caution about putting too much salt (sodium chloride) into a tank as a tonic. Especially where the water is low in minerals, that can cause trouble.

On another list (NANFA's mailing list) the issue of inexpensive brine shrimp hatching salt recently came up. I suggested getting that inexpensive livestock feed salt from agricultural supply stores. In this part of the world, it is simply sun dried sea salt and should do fine.

Others suggested taking plain old rock salt used to get rid of ice on frozen side walks and supplementing it with a little Epsom Salts (for the magnesium) and a little baking soda, to buffer the pH. Real purists could take a dash of calcium carbonate (another ice melter, but read the labels carefully) and toss a pinch into the mix, but it isn't necessary.

In fact, the plain rock salt usually does fine if you have a hydrometer. (Different sized salt crystals sometimes can confuse suggestions for dry measures.) Brine shrimp are quite tolerant of a wide range of salinities. A specific gravity of 1.02 is better than higher salinities, though Great Salt Lake brine shrimp can be found at specific gravities of more than double that. The baby brine shrimp don't tend to live as long with more minerals and why spend more for the extra salt? :)

The pet shop, where I purchase dog supplies, blackworms for the fish and some aquarium stuff, offers 10-pound bags of a popular marine salt. I'm  guessing that they buy it in bulk and re-bag it when business is a little slow. It may be that such salt is cheaper per pound than the usual marine packages and may be all a person would need for quite a while.