What is a RO/DI

[Kligonxx]

Ok i know or atleast think i know the RO stands for Reverse Osmosis no clue what the DI is and how important this is to a saltwater reef/fish aquarium. What does it do exactly? Why do i need one? Do i need one? What is the best one for a 100-150 gal tank?

 

[HN1]

DI is de-ionized. The need for RODI is that it gives you a "pure" known starting point. Add your chosen salt and you have the basis for everything. You'll also need RO for top off water (water evaporates, salt etc doesn't).

Which one you need depends on a lot including personal tastes. They are usually rated in gallons per day. We have a 50G and a 25G reefs and use a 50GPD RO unit. 10-20% water changes each week, probably 10G of topoff.

 

[Kligonxx]

how does it work exactly does it attach to the water line that you are going to fill the tank with or is it part of the filtration process that is constantly being used

 

[HN1]

They are multi-stage water filters that can screw into your tap or other water source. They are usually used outside of the tank system itself to make the water. RODI water can also be purchased, but having your own filter becomes cost productive pretty quickly.

Here's a generic diagram... there's a ton of info out there about them.
Google Image Result for http://joejaworski.files.wordpress.com/2007/11/diagram-of-reverse_osmosis_system.jpg

 

[Kurt_Nelson]

RO = Reverse Osmosis
RO/DI = Reverse Osmosis with Deionizing Resin

HN1 already covered a bunch, but the difference between RO and RO/DI is a matter of pureness.

The use of a RO membrane will take out about 95% of the junk in your water. So if you started out with water with a TDS (total dissolved solids) reading of 100 ppm, your water coming out would be around 5 ppm. Better than what you started out with... but still not "pure".

The use of DI resin after the RO membrane will take out the rest of the junk that the RO membrane missed. So with RO/DI, regardless of what you put in, you TDS coming out should be 0.0 ppm - as close to pure as you can get.

DI resin gets depleted, so it has to be replaced from time to time. How often you need to change it will depend on how bad your water is to start with. Best way to know when it needs changing is by using a TDS meter, and measuring the TDS of the water coming out of the unit, from time to time.

Whether or not you need RO or RO/DI is a matter of choice. It all depends on how pure of water you want. My opinion... if you're going to the trouble of making RO water, add the DI stage and know you're getting as good as water as you can possibly get. Why go to the trouble of doing RO, and just get 95% pure water. Who knows what's in that 5%!

 

[BigJim]

Think of it like the filter in a Brita pitcher, only better. Some piggyback on the water lines under a sink. Others have a dedicated line. Either way, it only filters when you open the valve.

I don't think I'd try to set up a reef tank without a RODI filter.