adding additives

It boils down to what YOUR tank needs. Every tank is different, so your gonna have to test and find out which element gets consumed the most.

 

I test for Mg, Ca, dKH and of course nitrates, nitrites, ammonia... the latter three are for testing for quality of water and the first two are to help maintain the levels needed for livestock. I use Tailored aquatics additives now after my tank crashed and everything is doing great!
TAILORED AQUATICS - Reef Magnesium, Calcium, Carbonate dKH Alkalinity, Kalkwasser Reef Supplements & Aiptasia control

 

i do agree with the idea of testing before you add, but the problem is most hobbyist test kits are not accurate.

in the case of iodine there is not a single accurate test kit out there, even lab testing is suspect.

Each tank is unique, in general water changes will replenish what you need...in cases where bacteria are used to reduce nutrients further, elements and other additives will be needed to prevent limitation.