Not sure where you are located, but in the Spring/Summer the quality of my house water decreases drastically. I am not sure why, partly may be temp-related. In short, TDS coming out of my tap jumps substantially starting in April and I burn through filters and resin much faster until winter returns. Sounds like your swapping filters this week, which I think is a good idea. Just thought I'd share another possible theory...related to the RODI idea just mentioned, but with a seasonal twist!