To answer the question about RO
It stands for reverse osmosis/deionization. It is a filter that does not go on your tank. It is a filter that pushes tap water through membranes/chambers that strips that tap water of TDS
--Total dissolved solids= nasty things that you don't want in your tank.
It is VERY important to the system and a great way to start out ESPECIALLY IF YOU EVER PLAN TO HAVE LIVE CORALS.
To sum it up, an RO
system provides you the cleanest water possible so what ever salt you are using to make up your tank water provides every thing that is in the tank water, without all the nasty stuff (phosphates, silicates, metals) that comes out of your tap.
is to make up your freshwater before you mix the salt in.