High-quality water is an important element of any successful aquarium. When used properly, RO/DI units will produce highly purified water free of harmful contaminants found in tap and well water. Mixing this purified water with a quality salt mix creates the high-quality saltwater essential for healthy corals, fish, and other livestock.
What is an RO/DI Unit?
RO/DI stands for Reverse Osmosis and Deionization. Most tap and well water contains a wide range of suspended particulates and dissolved impurities harmful to aquarium livestock. An RO/DI is a multi-stage water purification system that removes these harmful contaminants from the source water.
What is the Difference Between RO & RO/DI
An RO/DI includes an added deionization (DI) filtration stage while an RO does not. This additional DI filter removes significantly more contaminants than a simple RO unit alone.
How Do RO/DI Units Work?
At its simplest form, these units work by taking source water and passing it through a sediment filter, a carbon block filter, an RO membrane, and finally through a deionization (DI) filter. Each filter performs a unique function and should be replaced regularly for optimal performance.
The recommendations below are general rules of thumb. If you are making a lot of water, you will likely need to replace your filters more often. Proper maintenance and storage of your RO/DI unit will also help ensure your filters last as long as possible.
Sediment Filters
Purpose: The first line of defense in water purification, the sediment filter removes large particulate matter such as sand and silt.
How Often to Replace: Every six months or sooner if the water pressure after the stage drops.
Carbon Block Filters
Purpose: Carbon block filters effectively remove many contaminants, including organic waste, heavy metals, chlorine, and chloramines.
How Often to Replace: Every six months or sooner if the water pressure after the stage drops. Regular replacement is critical as chlorine and chloramines will significantly reduce the lifespan of the RO membrane.
RO Membrane
Purpose: RO membranes remove contaminants by forcing water molecules through a membrane at high pressure. This membrane removes organic chemical contaminants such as sodium, chloride, phosphorus, nitrate, copper, lead, magnesium, arsenic, and others, removing over 90% of all impurities.
How Often to Replace: Every 2 to 3 years or sooner if TDS readings after the RO rise. Most folks replace the other filter 4-6 times before changing the RO membrane.
Recommended Maintenance: Flushing, the RO membrane with a “flush kit” after each use, will extend its useable life. Many systems include a built-in flush valve or flush kit. We recommend purchasing a flush valve kit and adding it to the system if they do not.
Deionization Filters
Purpose: The final stage in most RO-DI units, DI filters, contains two oppositely charged resins that attract various contaminants. This step removes any remaining impurities and is a cost-effective way of producing highly purified water.
How Often to Replace: Every six months, if not sooner. Most DI resins will change color from blue to brown when they need to be replaced.
Comparison Chart for Common RO/DI Units
Below is a quick break-down of the components, costs, and recommended uses for three common units.
| Type of Unit Factors | 3-Stage RO | 4-Stage RO/DI | 5 Stage RO/DI |
|---|---|---|---|
| Suggested Use | Freshwater or Fish Only Aquariums | Soft & Mixed Reef Aquariums | SPS Dominant Reef Aquariums |
| Components | A Sediment Filter, Carbon Block Filter, & RO Membrane | Sediment Filter, Carbon Block Filter, RO Membrane, & DI Filter | Sediment Filter, 2x Carbon Block Filters, RO Membrane, & DI Filter |
| Source Water Compatibility | Well or City Water w/ Minimal Containments | City Water or Well Water w/ Minimal Containments | City Water w/ High Chlorine or Well Water |
| Initial Cost | $75 to $120 | $125 to $200 | $200+ |
| Replacement Filter Cost | Sediment + Carbon = ~$25 RO Membrane = ~$40 | Sediment + Carbon + DI = ~$50+ RO Membrane = ~$40 | Sediment + 2x Carbon + DI = ~$60+ RO Membrane = ~$40 |
Common RO/DI Features & Upgrades
While most RO/DI systems will come ready for use immediately out of the packaging, there are some upgrades that can improve the performance and efficiency of your unit.
Pressure Gauge
Measures incoming water pressure. The recommended water pressure is 65 psi or more. Also, the more pressure, the less wastewater produced by the RO membrane. A must-have, in our opinion. If your RO/DI unit did not come with a pressure gauge, we recommend adding one.
TDS Meter
Measure the total dissolved solids level in the water at a particular stage in the filtration process. Many inline TDS meters have two sensors; one to measure the TDS after the RO membrane and a second to measure the TDS after the DI cartridge. We highly recommend a TDS meter to help you know when you need to replace your filters.
Flush Valve
Another must-have is a flush valve kit that will “flush” out built-up deposits of minerals within the RO Membrane. This will help increase the lifespan of your membrane.
Additional RO Membrane
An additional RO membrane increases the speed of making water and reduces the amount of wastewater.
Booster Pump
Having ideal water pressure is one of the precursors to running a successful RO/DI system. Adding a booster pump is strongly recommended if the water pressure is below 65 PSI. Running the RO/DI at the proper PSI will improve system efficiency.
Drinking-Water Kit
Create and store drinking water with an RO or RO/DI unit. These units take water as it exits the RO membrane and store it for future consumption. Never consume water after it’s run through the DI cartridges as it is not suitable for drinking.
Additional Factors to Consider
Before purchasing an RO/DI system, consider how much space you have to operate and store the unit and how you will plumb the unit into your home. If you are tight on space, a smaller 4 or 5 stage unit is probably the best fit. Also, think about how and where you want to run the unit. If you have a large utility or laundry room, you may want to plumb the unit right into the sink. However, if that is not an option, most units can be connected directly to the faucet or an outside hose.
Alternatives to RO/DI Units
If you’ve decided on an RO/DI system, your alternatives are limited. Most local fish stores produce and sell their own RO/DI purified water and pre-mixed saltwater. This is a good option for smaller tanks; however, this will be significantly more expensive over time for larger tanks. While it may be tempting, we strongly recommend staying away from buying distilled water for the local grocery store as this water will still likely contain contaminants that are harmful to livestock.
We hope this article has shown what a valuable tool an RO/DI unit can be and how it can take your reefkeeping abilities to the next level. Having a water filter system, you can rely on will give you the peace of mind you need to focus on other aspects of reefkeeping.
If you have any additional questions about RO/DI or if you need a recommendation, please contact our support team.