The Importance of RODI Water in a Reef Tank

Initially when I purchased the tank I had the idea of using dechlorinated tap water mix my salt water and fill the tank. That would have been a huge mistake. But why not? I had used tap water for many years on my fresh water tanks. Any reef forum you visit you will see horror stories of huge algae outbreaks due to phosphates in the tap water.  A good salt mix will contain all of the trace minerals that you need in Reef keeping, so it is ok to start with completely sterile water for your mix.  RODI stands for Reverse osmosis dionization.  You will want a good 5 stage RODI unit.  Water purity can be measured in terms of "total disolved solids" (TDS).  A good RODI unit will put you very close to 0 TDS.  It should always be under 10 TDS.  I chose to purchase a unit that came with a free TDS meter.  This will tell me when I need to replace the filters or the DI resin.  You will want to test the TDS at each output of the various stages of contaminant removal.  This will tell you how your filters are performing and if you need to replace them soon.