Most companies recommend that you replace certain filters on your RO/DI water filtration systems every six months or so, but in actuality this time frame may not be useful to your situation. Instead of arbitrarily replacing pre-filters, DI resin, and carbon stages, look at the hardware to determine what’s really going on with the particular stage in question. When pre-filters get clogged, the pressure in the system will drop. It won’t be a big drop, but a noticeable drop nonetheless. To determine if the RO membrane is still working right, just look at the TDS reading after the filter. If that number increases above zero, then the filter is going to need to be replaced soon. The exhaustion of deionization resin, on the other hand, can’t be determined by hardware changes. Instead, you can look right at it. Most DI resins change color from a black/purple to a rust color when they get used up. The moral of the story is to pay attention to all of the different signals your RO/DI unit is giving off and make the proper filter replacements when need be.