Working on finishing up a write up on this very topic long overdue. I believe that the quality and purity of RO/DI is one of the most, if not the most overlooked essential elements to long term success. People chase numbers till there's no tomorrow scratching their heads when somethings not quite right, and throw every thing at the wall they can think of trying to solve it with little regard to the actual quality of the water they are using. Which I should add is often the heart of cause of their issues.
Definitely looking forward to that write up, I mainly think I'm fine as long as the water reads 0 TDS but I do feel like I'm always plagued by some mystery problem and always wonder if it has something to do with my water source.
Great write up on this too, I'm currently battling (no pun intended) too low of nutrients and having to dose po4 and no3...have more fish in the QT as we speak to hopefully help with more of a long term fix.