Sodium chloride is the familiar white crystalline material in just about every kitchen. Table salt as purchased at the local supermarket in this country also contains iodide and anti-caking compounds.
Iodine is a halogen, and is required for vertebrates in its ionic form. Iodine is the element; iodide is the ionic form. Do not confuse either of those with "tincture of iodine" which is a topical antiseptic and quite toxic. Iodide is necessary for our metabolism as an essential part of thyroid hormone, which is our metabolic pacemaker. Soils in wide areas of this country are deficient in iodine, and this lack can result in goiter (hypertrophy of the thyroid gland, effectively from insufficient iodide intake). Thus the practice arose of adding iodide to salt intended for human consumption. This was the safest (the levels of iodide are minute) and surest way of protecting the population from this deficiency as salt is ubiquitous (all but universal) in food processing and preparation. The levels of iodide added to table salt are so small that any water-living vertebrate or invertebrate would be pickled in brine well before toxic concentrations of iodide could be reached, so that particular urban myth is without foundation. In fact, a number of our tank inhabitants need iodide- most crustaceans have a significant demand for the material, and a number of fish can develop goiter in captivity from the lack of iodine- African Rift Lake fish seem especially prone to this. The often-discussed toxicity of iodide could be considered urban myth #1.
Salt, sodium chloride, is hydroscopic- exposed to air of more than Sahara-at-midday humidity, it will pick up moisture from the air on the surfaces of the individual crystals, which melt at the surface and cement themselves together- in short, they clump. The salt shaker does not work with clumpy salt, so additives are used to block the clumping. Arrowroot is common for this purpose, but others are possible. Again, as with iodine, quantities are small (but larger than with iodide), and are food-safe and fish-safe.
Kosher salt is commonly suggested as an alternative to table salt, as it does not have iodide added. This of course is a response to salt myth #1. Others gasp in horror at this suggestion, as kosher salt may have yellow prussiate of soda (the sodium salt of prussic acid, a ferro- or ferricyanide) as its anti-caking agent. Horrors! That is a cyanide compound! You are sending your fish to the gas chamber and it will kill them instantly! Horsefeathers. Once again, the quantity is tiny, food-safe, and the fish would be pickled in brine long before potentially toxic levels could be reached. Salt urban myth #2 down.
Tips on how to do this please? 
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