RO water

yes it is ready to go.but for a reef tank you want RO/DI water. RO mechcanically cleans the water by way of a filter. the DI polishes it off chemically.

 

depends what the TDS of your water is. usually 3 stages is the norm. i have a 5 stage sys,as my city water has a high TDS

 

I use a 100gpd no-name locally made Ro/DI unit that produces good water.

the brand name isn't as important as good membranes and pre-filters.

2/3/4/n stages are all additions of various pre-filtration. Mine has 2 sediment filters, a carbon filter, the RO stage, then 2 DI stages. It was sold as a 3 stage, so I work under the assumption that stages refers to pre-filtration, the RO. is implied? Maybe 3 stage refers to the "pre-filter" train, RO membrane, and DI stages - I'm not really sure.

 

agreed with pretty much everything that has been said above. you may be able to get away with tap water using your method with a low bio load tank and very hardy corals and fish but in the long term it will become harder to take car of algae and support corals. even the high end RODI units pay for themselves with in a year (depending on how much water is used). its arguably the most important piece to your aquarium jmo.
you can get cheapish units online and just fit them with good filters.
In terms of stages for a unit the more stages the less pressure is placed on your RO membrane and will increase its lifespan. a 3 stage typically means 1 sediment filter, 1 carbon, and RO membrane. a 4-5 stage usually refers to the addition to the sediment filters. the DI is sometimes included into the stages but just look at the unit description and see if it includes "DI"... RODI or just "RO"... no DI. i.e. my 5 stage includes: 2 sediment filters, 1 carbon, 1 RO, 1 DI, and is called a 5 stage RODI unit.

 

"Getting away " with tap water is a gamble most of us do not want to make with reef inhabitants, it is highly discouraged and will come back to bite you almost every time.
For now if you cannot afford your own RO/DI unit I would suggest finding a Water & Ice type store and befriending the owner. I have found even though the water is RO only and not RO/DI they usually do a good job of treatment and once they know you are using it for a ree fsystem they bend over backwards to test the water in front of you to assure of its quality. Avoid vending machines unless you have a TDS meter and use it each and every time you purchase water since machines are not governed by any health agencies and there are no regulations on water quality from them. Water store RO is 25 cents a gallon and you can but 5 gallon or larger jugs at WalMart and many plces very cheap.
Don't spend money on a drinking water quality system which will end up costing alsmost as much as a true reef quality system but will not live up to its claims. Save your money until you have the $149 needed for the ProPlus found here:
SpectraPure Customer Appreciation SALE! 20% - 50% off

For $149 you get the TDS meter already mentioned plus an inline pressure gauge, a tested and treated RO membrane, a 1 micron prefilter and carbon block, a full sized 20 oz vertical DI filter and much more plus its backed by Spectrapure a company in business for 25 years doing just this, not a fly by night ebay vendor with no brick and mortar building or physical address.

The word '"Stages" is a misued term often touted by ebay vendors to make their drinking water filter sound like a rocket ship. It means very little in that application since the "stages" they provide do little or nothing for a reef tank. Just because they have two carbons does not make it the same as a single 1 micron or less high quality carbon, nor do two little clear horizontal hollow tubes they call DI filters and hold about 6 oz of loosely packed outdated DI resin. Look for a RO only with 3 stages or better yeat a RO/DI with 4 stages, no more and no less unless you have very specific problems that need addressing with additional full size DI or silt that needs a different form of pretreatment.
The Pro Plus contains 4 real functioning stages an will outperform anything in its price range easily due to is superior RO membrane and DI resin.

 

ya you can use the ro water right away. you would wanna mix it with the salt and let that sit for atleast 24 hours. i aerate it and leave a heater in there too. as far as i know there really isnt a huge difference between stage 2 and 3 but im not entirely sure. i know a lot of people here got theirs at airwaterice.com just get the best one you can afford.