When maintaining an ecosystem of living creatures, we try to eliminate any outside possibilities of chemicals, pest, disease or anything else that could potentially harm your ecosystem. Because RODI goes through an extra stage of deionization it makes it more reef safe and less harmful for your tank. There are also many water conditioners that people add to their water that may make it "safe" but won't stop buildup of contaminants and compounds. We prefer RODI water because it removes outside factors, and this will also help with narrowing down any issue if dealing with tank problems. It is also a way of not adding unknown metal or other trace elements that can be extremely harmful to your ecosystem. Tap water contains metals as well as phosphates and nitrates that could lead to algae and other nutrient issues if added to your tank. If you plan to keep sensitive fish, SPS or even most LPS corals, RODI is necessary as it eliminates outside issues that could affect the water hardness, the salinity or the nutrients.