I just use RO water as it is now, and when I measured the RO water it wasnt exactly zero but close enough. There are some nutes in the soil for now I understand so I just give them the RO water for now.

I did some minor experiments on trying to reduce my pH 8 tap water to 5-6 or so by using chemicals and it didnt quite work out. The pH dropped but would go back up the next day, the ppms would increase by a lot... I found it to not be a good solution for plants. Better to start with zero and work from there in my opinion.  

So my question here wasnt for me but for him.