RO vs RODI Water
Reverse Osmosis vs RO/DI water for aquariums. TDS rejection, cost, when each is required (reef vs FW), and how to size a unit for your tank.
Side-by-side comparison
| Attribute | RO (Reverse Osmosis) | RO/DI (with Deionizer) | Winner |
|---|---|---|---|
| Output TDS | 5-20 ppm | 0-2 ppm | RODI |
| Required for reef tank | No (zero TDS needed) | Yes | RODI |
| Required for FW community | Optional | Optional | RO |
| Required for shrimp tank | Yes | Yes | Tied |
| Required for discus | Yes | Yes | Tied |
| Cost (100 GPD unit) | $80-150 | $120-220 | RO |
| Wastewater ratio | 4:1 (4 gal waste per 1 gal product) | 4:1 | Tied |
| Filter replacement cost/year | $30-50 | $50-90 (DI resin extra) | RO |
| Removes silica + phosphate | Mostly (95%) | Completely (99.9%) | RODI |
Frequently asked
Why does reef need RODI not RO?
Even 5 ppm input TDS contains silicate (drives diatom blooms) + phosphate (fuels nuisance algae). Reef tanks need zero-TDS input to maintain coral-friendly chemistry.
Can I drink RODI water?
Yes - it's ultra-pure water. But add electrolytes if drinking long-term (zero-mineral water leaches minerals from your body). Most reefers use it just for top-off + saltwater mixing.
Best RODI brand?
BulkReefSupply 6-stage 75 GPD ($175) - the hobbyist standard. SpectraPure MaxCap (premium). Aquatic Life RO Buddie (compact, lower output).
How often replace DI resin?
When TDS-out reads 1+ ppm. Typically 4-12 months depending on input water. Use an inline TDS meter to monitor (HM Digital DM-1, $25).
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