I just did mine a little while ago after being in service for a year plus. My TDS normally was in the low single digits but hadn't tested for quite a while. Getting a little bit of cyano so started testing and found my TDS had crept up to 20.
Was something how dirt and heavy the filters were and though the resin still showed a greeny clear from the outside, when dumped it out, the inside had pretty much had all changed to red/black.
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