I have a 9 year old Apec RO system. I also have a water softener. I have been pretty good about changing the first 3 filters since installing the unit. Earlier this year the water started having many tiny bubbles in it and there was a small bubble slick floating on top. The TDS went from the teens to the low 40s. Don't remember what the supply TDS was. Seeing all of the bubbles and the bubble slick I decided to replace all of the filters. Currently the RO TDS is in the high 30s and the supply TDS is in the high 300s. I replaced the filters several months ago and I still have tiny bubbles and a bubble slick floating on top of a glass of RO water. The water seems to go stale pretty quickly too. When first installed the RO water tasted wonderful, didn't go stale and was crystal clear. Any thoughts?