[e_baer]

well, if you use normal water from the faucet, it can contain nitrates and nitrites, also phosphates. and algae loves those things. so if you use an RO ir DI unit, it removes all of those things and makes the water pure water, and only water. so when you add salt to it, depending on what salt you use, calcium, strontium, iodine, iodide, and many ttrace elements will be added back to the water. then when you add it to your tank, it is cleansed water that is enhanced with minerals and whatnot.