All I can tell you is when mine started to creep up on the TDS meter, I replaced the carbon and filter media's (NOT the membrane) and it didn't change much. I then changed the membrane and all was good.
So in my experience the membrane would be it, at least to start. However the other ones are fairly inexpensive, so you can always do them all.
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Setup: 180G DT, 105G Refuge (approx. 300lbs LR, 150lbs Aragonite)
Hardware: Super Reef Octopus SSS-3000, Tunze ATO, Mag 18 return, 2x MP40W, 2X Koralia 4's Wavemaker
Lighting: 5ft Hamilton Belize Sun (2x250W MH, 2X80W T5HO)
Type of Aquarium: mixed reef (SPS & LPS) with fish
Dosing: Mg, Ca, Alk
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