yes you need plant specific additives....when you run your water through a ro filter you remove all elements good and bad...thus the need for calmg+...It is an additive by botanicare.
Just buy gh 3-part in liters to start with. Get some ph down. some calmag+, unless you are using tap, but distilled or ro is best.

If you use tap, then get gh 3-part for hard water... a small bottle of superthrive, maybe...

you will get a nice healthy grow. just use 5-5-5 for veg and then 5-10-15 for bloom, and that will get you through a grow. you can tweak 3-part so many way, that eventually you understand when to add or dilute strength. goodluck calmag values depend on your water, and you can add a 1/4 tsp per gallon of superthrive in veg also.