Quick take
RKIN recommends replacing RC-QT-CF every 6–12 months as needed. The current individual listing is about $45.43.
Role in the U1
RC-QT-CF is the fourth-stage post-filter used after RO purification to alter final taste/mineral profile.
Replacement schedule
RKIN publishes a 6–12 month interval.
Current price
The current replacement-filter collection lists the alkaline post-filter around $45.43.
Why final TDS may rise
Because this stage adds minerals after RO, finished-water TDS can be higher than membrane permeate TDS without indicating a failing membrane.