Reviewed 2026-05-01 · comparison
4 stage vs 5 stage RODI system comparison
Choosing between 4 stage RODI systems and 5 stage RODI systems is a fit decision, not a lab result: tank size, reef goal, budget tier, failure mode, and upgrade path decide which one belongs in the build.
Choose 4 stage RODI systems when
- Source water with moderate TDS and no known chloramine pressure where a standard sediment, carbon, membrane, and DI path is enough.
- Budget builds that can verify output with a TDS meter and replace cartridges before breakthrough.
- Small reefs where disciplined filter changes matter more than buying extra stages immediately.
Choose 5 stage RODI systems when
- Homes with higher TDS, chloramine concerns, or heavier water production where extra carbon/DI capacity can protect the tank.
- Larger reefs where water changes, ATO demand, and emergency mixing make source-water reliability more valuable.
- Reefers who want more margin before DI exhaustion or contaminant breakthrough.
Fit math
- Start with tap-water TDS, municipal disinfectant type, and weekly water volume before choosing the number of stages.
- The right RODI path is measured by output TDS, filter life, and contaminant removal, not by stage count alone.
- Budget for cartridges and meters because a neglected 5-stage unit is worse than a maintained 4-stage unit.
Risks before buying
- A 4-stage unit can be false economy if source water exhausts DI resin quickly.
- A 5-stage unit can be unnecessary spend if the tank is small and source water is easy to polish.
- No RODI unit helps if output TDS is not checked and filters are not changed.