Forget the word "stages" exists. Its a sales gimmick from the ebay vendors to make a chep system sound like the best thing since sliced bread. They tack three or four useless filters on and call it 5 or 7 or whatever "stages".
For a good reef quality RO/DI you want and need 4 good stages. One low micron prefilter say 1 micron or less, one 0.5 or 0.6 micron 20,000 gallon carbon block, one RO membrane and one full size vertical refillable DI filter, thats it.
There is absoluitely no need for dual carbons with todays technology. two carbons goes back to the days when carbon and prefilters were not as efficient as they are today. Many who use two carbons are also using a high micron prefilter which forces the more expensive carbon to act as a secondary sediment or prefilter which is a waste of money nad detrimental to the membrane since it also leads to lower entrance pressures.
Look at something like the CSP-DI on sale here:
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You get a 0.5 micron absolute rated (much better than nominal rated), a 0.5 micron 20,000 gallon carbon block, a treated and tested high rejection rate 90 GPD RO membrane, a ful size vertical 20 oz refillable DI filled with custom hand blended reef specific DI resin, an inline pressure gauge, a dual inline TDS meter and a capillary tube type flow restrictor.
Or the 75 GPD Premium here:
http://www.buckeyefieldsupply.com/showproducts.asp?Category=168&Sub=166
With this one you get a 1 micron prefilter, a 0.5 micron 20,000 gallon carbon block, a 75 GPD Dow membrane, a full size vertical refillable DI, an inline pressure gauge, a handheld TDS meter, a thermometer, a DI bypass valve and a capillary tube flow restrictor.
Both are excellent systems and will perform up to reef quality standards. They both use higher quality standard size replacement filters available most places.
Again more stages are not better, in fact in many cases its worse. Its what is contained in the stages that counts not how many.
