I read with RO water you need to be careful with cal/mag.
Stop reading.
It's a plant. It's actually a weed. It will grow. Don't overthink shit. Most people, I would estimate about 97%, post complete bullshit. It's worthless old wives tales for the most part.
I use RO exclusively. (I have a 5 stage RO/DI unit). The only thing you have to take into consideration with RO/DI is that it is 100% H2O. There is NOTHING else in it.
Plant in a 5 gallon pot with decent soil, add about 8 tablespoons of lime (a bag of it cost 5 bucks and will last you for a long, long time) and you can throw your cal/mag in the garbage. You'll not need it. Ever.
I spend a grand total in materials for a grow (I grow two plants once per year) of about 12 bucks. I usually have to buy a new bag of soil for each grow. That's it. I haven't bought nutrients in 4 years. ( I use Miracle Grow - regular formula I dose once with at the end of veg and tomato formula I use about 3 times during flower. Maybe 4.)
And the bag of lime I told you about.
That's it. That's all there is to it. I top them twice, and other than that leave them the hell alone other than watering when they need it...and ONLY when they need it. There is no schedule for that. Stick your finger in the soil. Is it dry? Water it. Is it moist? Don't.
People get into this thing and everybody thinks there's all this skill and magic formula and nutrients, nutrients, nutrients, blah, blah, blah.
Like I said, that's all I use man...that and my trusty 600 watt HPS/MH lamp. I net between 4 and 6 ounces per plant every time. My record: 8.6 ounces from one plant.
I do very, very little to them. They're weeds, man. In good soil, they'll grow damn near no matter what. The number one cause of every problem I've ever seen on this forum or any other is nutrient burn. People just read all the bullshit about nutrients and go all in with that shit.
Nutrients, in the end, is the number one cause of death of plants. People just think they're they answer to everything and that more is better.
They couldn't be more wrong.