The type of unit depends different factors. What kind of water supply will you be using, city or well? if city what type of sterilation do they use, chlorine or cholramines? In general the worse the condition the water is in when you start the more stages you will want for the best possible output water. Also more stages in a bad water condition may help extend the life of the filters, in that only the first couple of stages might need to be changed as opposed to all of them pretty reguraly in a couple of stage system.
The difference between the units at places from bulk reef supply and a hardware store is in the hardware store they are more geared for drinking water were as fish supply stores strip just about everything out of the water so you get the best possible water which is not usually recommend for drinking.
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