stands for Reverse Osmosis, and is a type of filtration done on tap water to make it pure. More or less, water is pushed through a really really fine membrane and the membrane only lets the pure water get through. People use RO
(normally combined with a DI
stage) to make pure water to add their salt mix to, and for freshwater top offs.
sterilizer uses ultraviolet light to sterilize the water. It doesn't filter it, or take out any heavy metals - it just kills anything alive in it. UV
sterilizers need regulated flow through them - if the flow is too fast, stuff isn't killed. People use UV
to help kill off disease or any nasty things in their water.