If you have enough stuff in your soil, and it's high in nutrients, you may not need to feed for a few weeks. Then when those nutes are used up you can feed however you normally do.
How low is your calcium? If it's just a little low, your tap water might add enough calcium to grow plants OK. If it's way way low, gypsum can be good. It's calcium and sulfur, and excess sulfur isn't usually a problem, so it should be safe.
Anyway, are you growing in your soil mix right now? How are your plants, are you having any problems?
