Everything you need to refresh your 200 GPD RODI system in one kit:

• 2 × 100 GPD RO membranes — the workhorses; replace every 2–3 years depending on your tap water
• 1L mixed-bed DI resin — replace when your TDS meter reads above zero
• 1 × CTO carbon block — replace every 6–12 months
• 1 × PP sediment filter — replace every 6–12 months, or when you see visible discoloration

Keeping up with filter changes is what keeps your water at zero TDS. If your tap water is high in sediment or chloramines, check the prefilters more often.