So ive been cleaning windows for 2 weeks, making 600 litres a day. My resins all ready needed changing (running 2x 25 litre tanks). Does this seem right? Expected it to last a lot longer. Tds of tap water is 60, first tank takes it down to 20, last tank 0. Thats about 60 quid a month on resin running at 60tds before it goes in?