RO vs. RO/DI


Relentless Question-Asker
I'm sure this has been asked before, but how important is it to use RO/DI instead of RO only?

For example, my tap water has only 20 ppm TDS.

My copper is .003 ppm
fluoride is <.01 ppm
nitrate is .03 ppm
nitrite is <.01 ppm
arsenic is <.0002 ppm
don't know phosphates, but I'm trying to find out.

Last edited:

Randy Holmes-Farley

Reef Chemist
Premium Member
That copper is measured where? By your water company? If so, it is not a useful number. Most copper comes from your own pipes, not the water supply.

That said, ammonia from chloramine in the tap water is a reason to use DI, as is silicate, which gets through RO fairly readily.


Relentless Question-Asker
Thanks for coming to the rescue, Dr. H-F. In other words, I'd have to test the water coming out of my tap, independently, to have a better idea what's in there. Those numbers come from the Public Health Department via the water company's engineering dept. Public Health tests the water. Oh well, it was a good idea, while it lasted. :)


I got nothin'
My tds on my tap is around 780-800. :(

Freaking WV water is rediculous. I won't even drink it.


Team RC member
Team RC
For marine reefs, RO/DI is desirable/critical. For drinking, RO (not RO/DI) is preferable.