on a small reef, the tapwater purifier is fine. (but for my system which goes through ~30gallons a week, it just doesnt cut it) it will be cheaper than buying water from a store, but will still cost more than an RO unit (in the long run) but that way you dont have to put up the initial cost of an RO.
if you intend to stay in the hobby for a long time, or think you'll be upgrading tanks in the future, i would strongly recommend saving up for an RO/DI unit.
i am a frugal reefkeeper, but getting an RO/DI unit is one purchase that i see great value in, and is definitely a purchase that made me say, why the heck didnt i get it earlier!
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