Reef keepers often mix saltwater from RO/DI water, which is tap water passed through a reverse osmosis membrane and then a deionization resin stage. The same water tops off tanks as the water evaporates. Systems differ in how many stages they have, how much water they make per day and whether they need a pump.
The six systems below range from a compact 50 gallon per day unit that mounts on a wall to a 150 gallon per day system with a pump. Specs come from each product’s listing page. We have not used these systems ourselves, and filtration claims throughout are the makers’ own.
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What to Look For
| Feature | Why it matters |
|---|---|
| Output (GPD) | Gallons per day sets how fast a mixing tank fills |
| Stages | Four stages is common; some add a sixth or a pump |
| TDS meter | Lets you read the water going into the DI stage |
| Mounting | Under sink, wall mount or a free-standing frame |
| Spare filters | Sediment, carbon, membrane and resin are all replaceable |
Specs at a Glance
| System | Output and stages | Install and size | Notes |
|---|---|---|---|
| Max Water | 100 GPD; 6 stages | Under sink; 15 x 6 x 17 in | Clear housing; TDS meter |
| Bulk Reef Supply | 75 GPD; 4 stages | Under sink; 15.5 x 6.25 x 16 in | Built-in TDS meter and pressure gauge |
| Aquatic Life RO Buddie | 50 GPD; 4 stages | Wall mount; 13.75 x 7.5 x 6 in | Compact and portable |
| NU Aqua Reef Series | 100 GPD; 4 stages | Under sink; 18 x 12 x 18 in | 3 year warranty stated by the maker |
| SpiroPure | 150 GPD; 4 stages | 4.4 lb | Mixed-bed indicator DI resin |
| iSpring RCC1DP | 150 GPD; 5 stages | 20 lb; tankless | Pump included |
The 6 Best Aquarium RO/DI Systems
1. Max Water 6 Stage RO/DI System, 100 GPD
A six-stage 100 GPD reverse osmosis and deionization system in a clear housing, sold with a TDS meter. The listing gives its size as 15 by 6 by 17 inches for under-sink installation.
Suits: a six-stage unit with a meter. Strengths: a TDS meter and a clear housing. Weaknesses: the listing’s material and capacity lines look wrong. Skip it if you want a pump.
The product details list the material as copper and the capacity as 100 gallons, which does not match a 100 gallon per day membrane, so confirm the parts with the seller.
View Max Water 6 Stage RO/DI System 100 GPD on Amazon
2. Bulk Reef Supply 4 Stage Value Plus, 75 GPD
A four-stage 75 GPD system with sediment, carbon, reverse osmosis and DI resin stages, a built-in TDS meter and a pressure gauge. It includes a garden hose connection, a sink adapter with several ring sizes and a canister wrench. The listing supports water up to 500 ppm TDS.
Suits: a monitored four-stage setup. Strengths: a TDS meter, gauge and wrench. Weaknesses: the lowest output of the larger units. Skip it if you need to fill big tanks quickly.
View Bulk Reef Supply 4 Stage Value Plus 75 GPD RO/DI System on Amazon
3. Aquatic Life RO Buddie 4-Stage, 50 GPD
A compact 50 GPD four-stage unit with sediment, carbon, an RO membrane and DI resin cartridges. It mounts on a wall and measures 13.75 by 7.5 by 6 inches.
Suits: a small tank or a tight space. Strengths: a small footprint and portable design. Weaknesses: the lowest output of the six. Skip it if you mix large batches.
View Aquatic Life RO Buddie 4-Stage 50 GPD on Amazon
4. NU Aqua Reef Series 4-Stage, 100 GPD
A 100 GPD four-stage RODI system for under-sink installation, with installation hardware and a manual. The maker states a 120 day trial and a 3 year warranty.
Suits: a four-stage system with a long warranty. Strengths: 100 GPD and a stated warranty. Weaknesses: the listing’s power line is unclear. Skip it if you want a meter confirmed in the details.
The product details list a corded electric power source and deionization as the method, which does not fit a plain four-stage system, so ask the seller whether a pump is included.
View NU Aqua Reef Series 4-Stage RODI 100 GPD on Amazon
5. SpiroPure SP-RO400AQD, 150 GPD
A four-stage 150 GPD system weighing 4.4 pounds: a sediment filter, a coconut shell carbon block, an RO membrane and a final mixed-bed DI cartridge with indicator resin.
Suits: a higher output without a pump. Strengths: 150 GPD and indicator resin. Weaknesses: the details we read did not list a TDS meter. Skip it if you want a built-in meter.
View SpiroPure SP-RO400AQD 150 GPD RO/DI System on Amazon
6. iSpring RCC1DP Tankless RO/DI, 150 GPD With Pump
A five-stage tankless system listed by the maker at 150 GPD, built with polypropylene and stainless steel and weighing 20 pounds. A pump is part of the design to keep it working at low incoming pressure.
Suits: low water pressure at the sink. Strengths: a pump and five stages. Weaknesses: the heaviest unit here. Skip it if you want something compact.
View iSpring RCC1DP Tankless RO/DI System with Pump on Amazon
Specs to Check Before Ordering
- The output in gallons per day against the volume you mix.
- Whether a pump is included and whether your water pressure needs one.
- Whether a TDS meter is included.
- The replacement cartridge sizes and where to buy them.
- The maker’s installation and flushing instructions.
Mistakes to Avoid
- Reading gallons per day as a storage capacity.
- Skipping the maker’s setup instructions.
- Ignoring conflicting lines in a listing.
- Ordering a system with no way to read output water.
Aquarium RO/DI FAQ
What does DI add to RO?
The makers describe the DI stage as a final resin polish after the RO membrane. See the starter tanks in our aquarium starter kits roundup.
Do all of these include a TDS meter?
No. The Max Water and Bulk Reef Supply list one. Check the others before ordering.
The Short Version
For a clear housing and meter, the Max Water. For a gauge and wrench, the Bulk Reef Supply. For a small footprint, the RO Buddie. For a long warranty, the NU Aqua. For simple high output, the SpiroPure. For low pressure, the iSpring.
