Using RO Water +Rep for answering.

When using RO water is there another additive you need to add on top of the nutes? CalMag or something? I've seen it mentioned in passing but can't find any definitive info on it. I just hooked up my RO system and plan on starting to use it tomorrow. So if anyone can answer it would be much appreciated, +Rep for sure.

It depends on your fertilizer choice. If it includes Magnesium you don't need to use it unless you see a deficiency appearing on the leaves. If it does not contain MG, you can water once with Epsom salts 1/2 tsp/gal or use the cal/mag as directed.

Agree w/ pointswest, is your tap water bad in your area? I use tap and have not had problems for years, just let it sit out for days first so everything can dissipate off of the water.

Ideally get a RO filter for your tap they are only like 100 bux and than you have a clean low PPM base - than you get to choose whats in your water and how much - my 2 cents

also pointswest answered your question accurately

Thank you for the quick response pointwest. Turdnugget I HAVE to use RO, tap water is not an option at my house. My water is jacked for growing. I live on a well and there is such a crazy amount of lime and calcium that it insta-kills plants. My water also wrecks havoc on small appliances. If I were to take a few drops of water in my hand and flick it at a wall, that wall would dry with a thin, flaky, white crust. I already got a RO system I paid $92.00 (w/ free shipping), I just wanted to make sure I wasn't skipping anything. Thanks again for the help guys.

If using RO on soil, I don't think you need to add CalMag as the soil "should" have trace minerals in it. As stated some fertilizers come with trace minerals added while others do not and assume it will come from the water or soil.
If using for a hydro grow, CalMag or the like is for the most part a must have.

I use Nursery water (w/minerals for human tots) from Wally World, 98 cents per gal and the girls seem to love it.

98 cents a gallon times 50 gallons a week equals 49 dollars a week. So for a 12 week cycle we are talking almost 600 dollars!!! ($588.00 to be exact). $600 just for the water... no thank you. Thanks for chiming in I appreciate it, but that's not feasible for me.