If you have a reef tank then:
Calcium, Alkalinity are a must. There are also magnesium suppliments, but a water change can help there.
If you have a protien skimmer, and run carbon:
you may want to get something like Kents Essential elements to add once in a while. But here again a water change will do the same thing.
Although they make alot of meds for our fish, if you get ick the prefered method of dealing with it is, hyposalinity. There are other meds for secondary infections, but I personally would prefer to purchase them fresh as needed.
To help with the appitite and vitimans:
Selcon is a good way to go, particularly if you are feeding brine shrimp which are not very nutritious.
If you have a problem with phosphates:
some type of phosphate sponge like SeaChems Phos Guard or Kents Phosphate sponge. These are tiny little beads that absorb the PO4.
With respect to adding and dosing all kinds of things to your tank try to keep it simple because sometimes less is more.
Thomas