Reviewed 2026-05-01 · buying guide
Best RODI system for a reef tank
Choose by fit math first: 75 gallons, mixed reef, balanced tier, and the actual category rule.
4-stage value RODI
Value lane: a 4-stage unit that's enough for moderate TDS with disciplined cartridge changes.
Confirm a TDS meter is in the plan; a 4-stage unit is only safe if you measure it.
5-stage RODI
Balanced lane: a 5-stage unit with the extra DI capacity most tap water benefits from.
The fifth stage helps only if your source water actually pressures the DI resin.
Premium high-output RODI
Premium lane: a high-output unit for larger water demand and fast mixing days.
High output still needs storage and a waste line that match the production rate.
Decision rules
- Start with your tap TDS and disinfectant type, not stage count — chloramine and high TDS, not the number of canisters, decide what you need.
- A 75 gallon reef making ~8 gallons of change water weekly needs dependable 0 TDS output more than an extra stage.
- Judge the unit by output TDS, filter life, and contaminant removal; budget for replacement cartridges and a TDS meter up front.
- Right-size the production rate so mixing day isn't an all-night job, but don't oversize past what your waste ratio and storage can handle.
Avoid these mismatches
- A bargain 4-stage unit when your source water exhausts DI resin in weeks — false economy you pay for in cartridges.
- A 5-stage unit on easy, low-TDS tap water where the extra stage just adds maintenance.
- Trusting any unit without checking output TDS — a neglected RODI quietly feeds the tank what it was meant to remove.