Reef Crystals is a brand of salt and magnesium is not really nutrition, it is ONE of the BIG THREE minerals hard corals use everyday and they almost always need to be added in controlled doses, often everyday. Calcium & Alkalinity are the other two elements that need to be observed and added as well.
First, read up on the topic here on RC. Then buy good test kits like Salifert or Red Sea to see where you are now. Then with the knowledge you have you can buy or make cheaply everything you need, and add it correctly. You will have to make a choice at this time on the method you will use - 2 part or limewater are the 2 most used methods.
If you are keeping only soft corals the use of good reef grade salt and regular water changes may be able to keep the levels stable because they use less of it. But yes, you will have to keep levels, or "parameters" of these 3 items at certain levels and more importantly stable if you wish to keep LPS & SPS corals. If this is your plan it is a huge advantage to get THE BIG THREE right before adding hard corals. If you learn as you go you will kill stuff for sure.