depends on the size of your tank and what you have in it.
If your tank is 180g or more, and you have a lot of stony corals, then I'd suggest a calcium reactor.
I have a 200 with mostly softies and a 150g with just a few LPS and SPS and I dose both of them by hand. When the corals get bigger I will be wishing I had a calcium reactor.
When my 200 had very large SPS colonies I was dosing about 150ml each of calcium and alkalinity per day.