I have a 450gpd ro system.
I work with a tds reading of 000 upto 003 and then look to change .My reading was 003 so i changed my resin and the reading was 001 .
I use to change the prefilters and resin all at the same time at a cost of £30 every 2 months , until this magic forum advice told me to just change the resin .However still 001 .         So i FLUSHED  for a hour and now the tds reading is 000.

This makes me realise is FLUSHING a priority every month for a hour or so /