When getting into the hobby of marine tanks, the basic consideration is water. Without it, nothing in your tank would live for long. Compared to freshwater tanks, marine tanks need excellent water quality to maintain success. So what water is available to the average person?
Tap Water - anything can be in your tap water, and most are undesirable for your tank.
Well Water - similar to above, with the risk of metals and high alkalinity
Distilled Water - available in many food stores, supposedly pure.