Fertilizer and Mag/Ca Questions

[Christine1227]

Hi! A bit of a newbie here, but have lurked for a very long time. I'm unsure where to post this question, so my apologies if this is section is incorrect!

I have 3 phals, a vanda, an oncidium, a mini cattleya, a den phal, and a cymbidium in my current collection. My question is regarding fertilizer and mag/ca. I water with tap and understand that tap water generally contains enough mag/ca for plants to survive on. However, do people general supplement additional mag/ca, even if they water with tap?

To piggyback off of that, if I was to purchase the MSU pure water fertilizer (13-3-15 8Ca 2Mg), AND use tap, would it still be suggested to get a mag/ca supplement?

Thanks!