water is very pure water, the impurities of tap water and sometimes well water and sometimes even distilled water can causer algae to grow unchecked in a reef tank. If you are not going to start a reef tank with high intensity lights, then it is unlikely that you need
filter, although you would almost certainly benifit from it. Test your tap water for PO4
, Nitrate, nitrite and ammonia, might also check the CA
, some people have very good water coming straight from the tap. For good prices on RO
units check ebay.