Test anything you think you may need to supplement. Don't just add it. Test it and find out how short you are. A high quality salt used for water changes on a regular basis should provide almost everything if not everything. If you happen to have enough corals that you do genuinely need to supplement things like Ca, Mg, KH, etc. then you need to use a high quality test kit.
There are MANY supplements out there. Many are not needed or have circumstantial support at best. Most are not needed.
Let's go with a hypothetical. Let's say your KH is too low. You test it and find it is 6 dKH instead of 8-11dKH. Test the water that you use for water changes. By testing this you will see what the starting point is and see if it is the salt that is deficient or if the tank is actually using it at a higher rate than you are adding it. If it is the salt that is deficient you may need to change salts. I use Tropic Marin (regular, it actually has more KH, Ca, etc. than the Pro Reef). If you decide not to change salts or it is not the salt then you need to supplement KH. You now need to decide if you want to use a powder or liquid supplement. You will generally get more per dollar with a powder, but you should premix it in water (at least if you are not adding it to the beginning of the sump). Many KH are part of a two part or A and B supplement that also has a separate Ca supplement that you may not need. I don't suggest these. Even if you need to supplement both it is unlikely you will need to do so in a 1 to 1 proportion. Now that you have your product it should say how much it will increase KH in a given volume (for example: 5mL will raise dKH one degree per 50 gallons). Start with half of what you think you need. For one you don't want to overshoot. But you actually don't even want to get it dead on right away. You want to slowly change chemistry, quick changes can shock corals. Test and add the supplement daily if not twice daily. Ideally track it on paper or on your computer (with something like Excel). This will give you a perfect picture of what your tank does when you do a water change, top off, etc.. Eventually you will figure out that every day you need to supplement x amount of supplement. At this point you don't necessarily have to test daily, but you should still test on a regular basis to ensure you aren't under or over dosing or that the needs of your tank haven't changed.
Avoid things like iodine, trace elements, amino acids, coral boosters, etc. Almost none should ever be needed as a supplement and adding them can either be a waste of money or even harmful.
