When should I change my RO/DI filters?

[SaltyMember]

I have a RO/DI unit for almost a year now. I have heard that they should be changed at least yearly and I am planning to do so in a month or two. But, I tested my TDS today and got a reading of 4 ppm? Does this mean I should change the filters now or can I wait a little longer. I used to get readings of 0, but does it need to be changed if it no longer reads 0?

[Steven Pro]

I generally change my prefilters every six months. See here for additional information http://www.reefcentral.com/modules.php?s=&name=Content&pa=showpage&pid=2

[SaltyMember]

thanks, that is an excellent article. Guess it's time to change the prefilters.