Depending on your tank inhabitants and your goals you might benefit from using the 4th pump head for amino acids like Fuel or Acro Power. Without knowing anything about the tank it's hard to say. I would start by answering the question why do you want to dose trace elements? Are you looking for better color, growth etc. It's a good idea to be testing for anything you are adding to the tank in concentrated form like calc, alk, magnesium, and trace elements to see if you actually need to be adding them and to ensure that you don't overdose the tank. There are test kits for the major elements, some easier to read than others.