If you have good tap water stick with it (< 200ppm), RO water will just decrease the amount of PH up and down you use since the buffering capabilities are gone. As far as RO PH...around 7 for sure for pure water, now since its pure and has no buffering capabilities any small amount of an acid or base will cause a huge swing.

Cl doesnt break down...its an element, it will leave the water on its own in a gas state, any carbonic acid in your water will be made from CO2 in the atmosphere reacting with the pure water.

DI is just the last step in super pure water....you or your plants wont notice a difference....FYI DI water is so hungry for shit to dissolve it will eat though 6mils of stainless steel every year.

Your taking everything out of the water(almost) when you run it in an RO system so you have to add everything in turn...its just better control and you know exactly whats in your water.

Oh and TDS is just a measure of conductivity, so its really not measuring the total dissolved solids really accurately. Take some tap water and measure the TDS, now take distilled water with some PH down in it...doesnt take much. That PH down will cause the pure water to conduct electricity much better than the tap...your pure water may even read much high on your TDS meter than the tap even tough the amount of TDS is less.....temps can cause the same effect....you will notice conductivity (TDS reading) will drop in a linear fashion with temperature.
Cyclonite Reviewed by Cyclonite on . R/O water systems as more time goes by i start to realize how important it is to use good water when growing. seems as tho every serious hydro grower uses R/O water. which leads me to my question. what type of R/O system do you have 3 stage, 4 stage? it is a portable system or a permanent system with res? how many gpd does your system filter? also do you also use a DI filter? i have been searching on fleabay for a system. looks like some good prices but not sure on the unit i need. looking at a Rating: 5