ro/di unit-which cartridge fails first?

[ScienceAchieved]

I've have a aquaticlife ro buddie 50 gpd unit. My tap water is around 150 tds, and ive probly run about 200 gallons through it over the last 3 months. the di unit is only half used up. I was wondering what cartridge typically needs to be replaced first?
also, i should add that the first 2 buckets I made were 000tds. I noticed one of the tubes was pinched, straightened it out, and when I put it back down I ran it horizontally instead of vertically like usual, which is when I started getting the abruptly higher reading of 17 tds

 

[Browndawg80]

I typically replace the sediment and carbon every 6 months regardless of the TDS reading and change the DI as soon as my TDS start reading 001. I would highly recommend an inline TDS meter for about $30 if you aren't using one already. Good luck

 

[Jeff210]

I typically change all filter cartridges when I notice the pressure coming out of the cartridges and into my RO membrane drop. I find it is easier to just change them all at once as opposed to trying to remember which individual cartridge I replaced when.

I have the color changing DI resin so it is much easier to determine when that needs to be changed.