Fish supplements are substances added to the fish tank in addition to the normal food to ensure proper development and health. Supplements are usually used in reef aquarium containing corals and other shelled invertebrates.
Calcium is a very common supplement as it helps build the shells or snails, crabs, shrimps, etc.
Iodine, iron, magnesium are all important elements which are provided as supplements.
Garlic is an example of a common aquarium supplement. Garlic helps the immune system of fish fight disease. Food can be soaked in these supplements or the supplements can be added directly to the water.
Many ready made foods come with the necessary supplements already added.