an RO/DI unit has numerous benefits and it significantly improves the quality of tap water. An RO unit works by passing water through a very fine grained membrane at rather high pressure. What results is that rather pure water is all that passes through. The RO unit removes about 80% (depending on a few factors) of dissolved solids from the water such as copper, chlorine, silicates, all things that are harmful to a reef tank.
Then if your unit has it, the DI portion (deionization) passes the now cleaned RO water over a resin that removes 99% of the remaining dissolved solids, in particular, phosphates.
Now what does this all mean? Obviously the purer the water, the better right, and as far as technology stands, an RO/DI unit is the most economical way to do so. In the longrun, an RO/DI will prevent algae break outs such as diatoms and hair algae, both nuisances that plauge many reefers, and are difficult to rid.
There are alternatives to purchasing an RO unit. You can usually buy RO water from the store, but as you can imagine this sucks to carry bucket after bucket of water back and forth. Sometimes, grocery stores sell RO water, but these units are not always properly maintained which can affect the quality of water.
To sum up, RO water is almost a must. you can use tap water if you wish, but if you are serious about reefing, i PROMISE you that you will kick yourself in the teeth for doing so.
This site sells RO units as well as many others. ---- often has good units as well from an "h20 guru" see what he has to offer for the best bang for you buck.
HTH
