Tap water filter is a deionizer. Reverse Osmosis is a mechanical filter that will remove about 95% of impurities from tap water. Not good for Phosphate or Silicate but really good for almost everything else. If you run the output of a RO
through a DI
then you'll remove almost all the Phosphate and Silicate as well.
beads (rosins) are expensive so using a RO
first cuts down on expenses.