So, about 3 weeks ago I got my first ever RO setup. It has 2x 20" pre filters and a 20" HF4 membrane. My water TDS is about 220 and it's coming out of the RO at 007. I'm doing a 10 minute flush once a week. I'm making about 300/350 litres of water a day. How will I know when to change any of the filters and membrane?