Advice on RO DI

[salt newbee]

I have a few RODI (RO/DI, RO DI) questions???
1) What is the difference in a RO unit and a RO/DI unit?
2) What is the difference in 2, 3, 4 stage units
3) Is Premium Reef 50gpd 4 Stage RO/DI Unit a good unit?
Thanks much!!

 

[joncat24]

for what it is worth ro/di is more pure than ro. the more stages the better....I have a coralife 4 stage 50gpd pure floII ro/di and you can get it for $149 online. I have not heard anything about the one you mentioned.
alot of people buy the airwaterice brand also

 

[acrylic51]

What membrane are they using in their units? Rejection rate of membrane as well?