Hey ladies and gents!

starting a fresh grow as I had all males last time for my first ever grow

I’m germinating right now but I’m switching one major component.
I’ll be using RO water from start to finish this time instead of my 350ppm tap water that I didn’t know was bad lol I switched to RO when the plants were in week 4 and they were fine.

however this time it’ll be straight RO.
My question is should I use cal-mag from the getgo? Or should I mix RO with tap water to get around 150ppm until I start using nutes at week 3-4?
Growing in Promix HP