There seems to be a lot of probs caused when people change their RO prefilters.
Hopefully this bit of info will help.
When you change them ALWAYS FLUSH your RO system out after putting the new ones in. The reason for this is that there will be minute particles come out of the carbon filters (Called "fines") initially until they are cleared away.
If you dont flush these will get into your RO membranes and cause probs. Usually a rise in TDS out with subsequent increased DI resin use.
Better still is to disconnect the pipe fromthe prefilter housings into the RO and just let the water run freely through for a few minutes. Next time you change your prefilters try running the water into a white container and you will see these fines as very small dark particles.
I hope this will help you.
Andrew