Although only mandatory for reef tank with lotsa hard corals, I recommend the diy 2 part to maintain calcium, alk, and magnesium.
It's easy and very inexpensive.
FWIW water changes will limit but not correct increases and decreases in things due to the bioload in the tank. At least at change levels convienent to hobbists. Like 10% per weeek or so. If you could change 100% of the tank every hour and had access to perefect replacement water then yes you will maintain parameters through water chages. But at 'normal' water change levels you need to take other actions to maintain the environment.
Which is why reef tanks do have to dose for calcium, alk, and magensium
my .02