water means pure water that you will now add salt (and trace elements) to so it aproximates natural sea water. If you do partial water changes often enought that is all you need to do. Most of us do not want to do 20% or larger PWCs once a day or more, so we add items that beneficial bacteria will be able to attach to such as porous rock, sand beds, etc. If you have enough live rock and substrate you will get enough bacteria growing and living that will eat the ammonia produced by fish waste, uneaten food etc, into nitrtite. A DSB
, sump or refugium with macro algea or PWC
's will remove the nitrates.
A protein skimmer will remove SOME of the disolved organics. No one item can be used by itself, unless you want to be a slave to PWC
So NO, you can't really do just one. Also you want a RO
unit not just a RO
unit. Tap water can have 80 -300 ppm TDS
(Revers Osmosis) will cut that down to the 10-20 range depending on where it started. DI
(deionization) will cut that down to zero.
Hope that helps a bit. Sorry, this is an expensive, but ENJOYABLE hobby to start.