Ro vs ro/di

[Steve Dillon]

Ok

 

[AZDesertRat]

There are many things RO is not good at removing such as nitrates (actually all forms of ammonia including nitrates, nitrites and ammonia), phosphates and silicates to name a few of the common ones which can cause problems in reef aquaria. Most if not all of those are weakly ionized so do not register well on a TDS meter so may be present and you wouldn't know it.
DI adds an additional filtration step to eliminate those things that the RO misses.

 

[Steve Dillon]

That makes sense. Thank you for the info