One thing to consider is that you should have a test kit for EVERYTHING you dose. If you're adding magnesium make sure you have a test, so on and so forth.
Typically your partial water changes should be replenishing all the trace elements used by your corals, and dosing is not necessary. Dosing may become needed if your corals are using the trace elements faster than they can be replenished or if your salt mix ( ie: tropic marin reef crystals) doesnt contain any trace elements.
Hope that cleared things up a little. I have a tank full of corals, including SPS
and I havent been dosing and I'm getting astounding growth out of them.