Leave it to Russ. Man, that guy's an encyclopedia of info.

Here's the simplest way to see if the RO unit is filtering correctly. Order a TDS meter. They run from $15-$40 depending on the type/brand. A TDS meter just measure any impurities in the h2o.

Then get a chlorine test kit from your lfs or pool supply store. If you're getting a reading of 0-10 on the TDS and there's no chlorine detected, then your RO filter is working great.

Good rule of thumb is to replace pre-filters every 3-4 months and your RO filter (the expensive one) every few years.

Buckeye really is a good company and they have the cheapest prices on filters, that I've seen. Plus, they have a very helpful FAQ section that covers a lot of diff. topics.