Dosing: What should I test for?

I have a 75 gallon tank that has been up and running for 2 years with no problems. The goal for the tank was to have primarily soft corals, a few fish and a number of small critters to keep it interesting . Up until now I have not tested to see if dosing may be beneficial.



The question is, should I be testing for anything or do you think simple water changes are enough to replenish what is needed? I'm getting conflicting answers since this is a soft coral system only.