ray22,
there are many additives that can benefit a setup, but should be determine by the target setup (fish bioload, species kept, ect.)
for example if you are having problems with phosphates, then a phosban reactor can help you achieve colorful corals, not keep them alive and brown.
in some setups magnesium is low, and it can help make it harder to keep alk and calc in check, also its used in the tank
in other cases there is no to little coraline algae (if user wants it than thats a problem else no) many have seem explosive growth by supplementing with strontium,
have can't get their nitrate/phosphates down, and try vodka (with proper knowledge) and vodka provides a carbon source for the bacteria, and they explode in growth, eating more nutrients, but that bacteria needs to be skim out via a good skimmer. ect
many report good results while other said is just snake oil and doesn't make a difference with products such as salifer/kent amino acids / essential elements, iodine, iron (has been shown to be good for increasing macro algae growth), ect.
now there are filtration systems such as zeovit, prodio, which are bacteria driving and thus is added to the tank, these have followers with impressive tanks, and haters
with impressive tanks also that said that is just snake oil, or people like jda above with comments such as "I also don't use anything where the manf won't disclose what is in their product."
and jda no offend intended
and on and on ray22
good luck,
sam