additives and food.

Additives are not a must for keeping corals healthy.  The trace elements that the corals need to thrive are the more important topic.  If you do frequent water changes then it is not parative that you use additives, as this will usually add them back into you tank.  Also you do not have any high calcium loving corals, so you probably don't even need to worry about adding calcium to the tank.  But to help speed up their growth rate some additives work great for this.  However, when ever dosing additives into a tank it is important to measure them as to not over dose, as this can cause serious problems.

One thing that you mentioned is feeding your corals.  Additives are not a method of directly feeding your corals.  Additives are like vitamins to us.  To feed your corals there are several things on the market you can use.  The most common are Rotifers and Green Water.  Some corals will even eat small shrimps and such.  So that is what you would need to 'feed' your corals in addition to proper lighting.

Marine Snow and Black powder are coral food. If sufficient filtration is available, feed a lot and watch the coral grow.

One of our members, Ross Bagshaw, whose tank has been on the reef tour for the last three years, asked me once what he could do to slow his coral growth because everything was growing so fast it was squeezing up out of the tank  I suggested that he reduce his regimen of additives which included lots of so called "vitamins". He is religious in his addition of a plethora of additives. His tank is in the reef tour section. It's the 120 where you can hardly see anything but coral. Ross is the lucky older gentalman who's wife always brings such nice goodies to the WMAS meetings.

In short, additives are not necessary but do make a difference.