Hey you are probabably tired of hearing it - but knowing the parameters are a must. "Single most important addative" question is a no brainer to me! Water ghanges! On a regular basis this will bump up your trace elements every time you perform one. Calcium and alkalinity will both be "consumed" by your tank and need to be replaced at least weekly. Day to day biological functions within your tank want to make the water more acidic (lower pH), your levels of both alk and ca will try to prevent this. They "buffer" the water against changing. There is a wonderful thread in the archives relating how pH, ca, alk; are all tied together and dependant upon one another. That is the reason so many answer this thread wanting to know parameters not "within spec".
Within your thread you mention a "leather" died first; could it have been one called "devils hand"? If so they are normally toxic to most sps corals but if it died and released everything into tank this could be very toxic.
Lighting - you mentioned power compacts which may or may not be okay for the corals you have. Most smaller tanks forget volume - think depth of water, will be fine with said lighting, if your tank is over 20" deep there may not be enough punch to penetrate.
Everything goes back to knowing what your system contains. Skimmer, sump system, filtration, lighting, flow rates, and parameters like - temp, pH, amn, nitrite, nitrate, calcium, and alk at least.
Good luck and when you need helps these boards can be very useful as can the archives within them!