Ion exchange (DI) resins
Final stage of the RO/DI system. They remove the last traces of impurities from osmosis water, bringing the TDS to zero.
Detailed explanation
DI resins (deionization) are the final stage of the osmosis system. After the RO membrane (which removes 95-98% of the impurities), the DI resins remove the remaining 2-5%.
Ion exchange resins work by exchanging ions in water with H+ and OH- ions. The result is ultrapure water with 0 TDS.
Resins change colour when used up: from blue/green to yellow/amber (in resins with colour indicator). When the colour changes the resin must be replaced.
The duration depends on the TDS of the incoming water (after the RO membrane). With a good RO system the resins last for months. With an old membrane they wear out in weeks.
Regenerable resins (with HCl and NaOH) exist but for home aquariums disposable resins are more practical and safe.