ro units

Most people that have reef tanks have RO/DI units.  When I purchased mine, I thought about how much I was willing to spend
and what I wanted it to do.  Then I let those factors drive my decision.  Some people have theirs hooked to their refrigerators as well as their
reef systems.  Of course, you need more stuff and it costs more, but
you get more of a benefit that just to your reef tank.
The first unit that I purchased, I bought used.  It was a hard lesson learned as even for the little that I paid for it, I found out that the DI resin was exhaused and was not a refillable resin container.  The good news was that the filters and membranes still seemed OK, but I barely used it and I'm going to be getting $20 less for it than what I paid for it.  Buying them used can be risky depending on how long they have been used.  Replacing filters and membranes is about $40 and resin is about $40 for 5 lbs which will fill up the canister about 4 times.
If buying new, Air, Water and Ice (www.airwaterice.com) is great to work with and gives 10% off to club members.  It also depends on what you want to do with it. 
Also, if you buy used, make sure and get a good idea of how much water has been run thru the filter to estimate about how much life is left in it.  If you can borrow a TDS meter, test the water that goes into the DI ccylinder.  That should be under 25.  The water that comes out from the DI cylinder should be as close to 0 as possible.
I  hope this helps.
--AJ

 

I bet you could find a used bare bones unit for a good price on ebay or from one of the forums like this one, then just buy new filters and membrane to go in it. Places like airwaterice.com and the filter guys have very reasonable prices on modest sized new units as well. It all depends on how much capacity you'll need.

 

i have the ro unit we raffled off at the frag swap I would be willing to sell pretty cheap. It doesnt have a di unit on it but one could be added. PM if interested

 

I am far from an expert in this field, but will add my 2 cents. I got one of the reefkeepr III by air water and ice and (after a handful of question being answered on this forum) I love it. Also got it on eBay with brand new filters and resing for 50 bucks, so I would take a look on eBay for one before buying elsewhere. I added a 3 gallon under the sink holding tank with a spigot that I got from air water and ice for 30 bucks bypassing the DI. Up until now we had been sing britta and are happy for the extra fridge space! Best tasting water ever, too. One of my few reef purchases that my girlfriend was actually happy with! LOL

 

Here's a link to a good RO/DI FAQ on Reef Central - http://www.reefcentral.com/modules....page&pid=2