In a marine aquarium we are just trying to replicate the natural eco-system. In nature there are tons of chemical compounds or elements which end-up mixed in the ocean water. Now this natural balance will not happen in the aquarium on it’s own.
These chemical compounds are the basic building blocks for the growth of marine life. One of these such element is calcium. We all know the importance of calcium in our lives. 
We consume calcium in form dairy products such as milk and yogurt also in our vegetables we have traces of calcuim. Similarly the marine life requires calcium for its growth, now what do we do, we can not add milk to our aquarium and the same goes for other elements such as 
magnesium, iodine or even iron, here these additives come in handy. When we start a new aquarium some of the compounds are present in the salt that we mix but as soon we add live stock and they start to deplete. To cover the gap we need to top-up the lost nutrients.
Dosing these additives or chemicals would make 
our hobby little more interesting. They are easy to use as most of the additive packing would indicate the amount to be mixed in water ratio. Do not forget to test the aquarium water before adding any of the additives. With out knowing about the system it will lead to tragic effects.
There is a huge selection of them available in the market from different brands to different in sizes to different elements. Be wise and use them as without the building blocks in an aquarium, it is as good as empty. If by any means anything goes wrong or there is a slight chance over dosing of these additive, the best way to remove them is by means of water changes.
