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Best RO DI System for Reef Tank: Buyer’s Guide

Are you in the market for a RO DI system for your reef tank? If so, you’re likely wondering what the best option is. In this buyer’s guide, we’ll walk you through the different types of RO DI systems and help you decide which one is right for you. We’ll also give you some tips on how to choose the right size system for your needs.

Just want information on our favorite RO/DI system for a reef tank? Check out the SpectraPure MaxCap RO/DI System. So whether you’re a first-time aquarium owner or a seasoned pro, read on for all the information you need to make an informed purchase.

Best RO DI System for Reef Tank Options:

What is the best RO DI System for a Reef Tank?

We think the SpectraPure MaxCap RO/DI System is the best RO DI system for your reef tank. We love this model because it has top-of-the-line RO DI filtration which is great for large or heavily stocked tanks. This system comes two 99% rejection rate RO membranes, pressure gauges, dual TDS meters, and an upgraded DI resin. It is also super efficient, meaning you’re not going to waste a bunch of water in the pursuit of clean water.

What is a RO DI System?

A reverse osmosis deionization system, or RO DI system, is an efficient filtration tool that produces purified water for saltwater aquariums. It removes contaminants from tap water making it safe to add to your tank. By forcing water through a special material called a membrane at high pressure, the membrane only allows the cleanest water molecules to pass through while blocking contaminants. The deionization stage brings the water down to 0 TDS (total dissolved solids). 

RO systems can produce drinking water and purify water safely for freshwater aquariums. RO DI systems are used to produce water for saltwater aquariums.

Why Do I Need A RO DI System?

A basic rule of keeping a saltwater tank (as basic as making sure your water contains sufficient salt, making sure you have sufficient water flow, and making sure you feed your corals correctly) is to only use water from an RO DI system for your tank. Don’t use straight tap water. The water is clean, and it won’t pollute your ecosystem with harmful chemicals like silicates, heavy metals, or chorine. If your RO DI system doesn’t come with one built it, you might also want to consider a UV sterilizer.

RO DI system for reef tank

While you can purchase RO DI water from local aquarium stores, it is actually cheaper in the long run to have your own system. Plus, if you have any random accidents or leaks where you need water quickly, you will have the ability to produce what you need instead of having to run to the store or keep jugs of water in your house.

Why does Unfiltered Tap Water Cause Problems in an Aquarium?

Tap water contains chlorine and chloramine which are harmful to fish. These substances are added by city water departments to prevent potential bacteria outbreaks.  If fish are exposed to chlorine and chloramine, they will experience skin and gill irritation.

Tap water can also contain phosphates, nitrates, and silicates. If these are not removed before adding to your tank, these elements can support unwanted algae blooms. Tap water can also contain copper and iron, which are not good for most reef animals.

Tap water is also likely a different pH than your tank water, meaning if you add large quantities, you may inadvertently shift important water parameters.

How to Select a RO DI System for a Reef Tank

There are several factors to think about when deciding on the best RO DI system for your reef tank. We break down the major factors below.

​​Quality of filtration is not completely tied to produced TDS levels

TDS (total dissolved solids) levels are important in an aquarium. However, there’s an idea circulating if your reef tank contains anything more than 0 TDS, everything will promptly die. And therefore, your RO DI System’s only safe output can be 0 TDS. While this is mostly true, it is not 100% truth.

TDS measures total dissolved solids but it doesn’t specify what the type of dissolved solid is. For example, water that contains 10 ppm (parts per million) of magnesium could be beneficial to your reef tank. However, if you had 10 ppm of copper, your tank could crash and your aquatic life would suffer.

Reef aquariums are negatively impacted by heavy metals such as arsenic, copper, and lead. They will also suffer in the presence of chlorine, chloramine, nitrates, and ammonia, which is why you can’t use unfiltered tap water.

A good RO DI system will produce reef-safe water. However, that does not necessarily mean it will contain less than 1 ppm of TDS. That is not the ultimate sign of quality for the system. What is most important here is that your tap water is purified through a RO DI system prior to use and contains a max of 30 TDS prior to use.

We also recommend investing in a quality TDS meter, like this option. This way you’re able to easily test your water before adding to your reef tank, giving you another element of control.

Deionization stage is a plus (but not necessity)

Deionization units “polish†the water as the last stage of the process by bringing the TDS levels down to around 0. In other words, the RO portion handles 99% of the work and the DI captures the remaining 1%.

Deionization units use charged ions to capture contaminants, specifically iron, copper, silica, and nitrate ions. These are pretty gnarly elements in your tank so if you want to be super safe, we recommend a system with both RO and DI capabilities.

However, if you end up choosing a system with just a quality RO system, you’re still going to capture most of these problem elements.

Efficiency of GPD rates

Make sure you pick a RO DI system that can support both the gallon volume of your aquarium and your stocking levels by selecting a model that can provide your needed Gallons Per Day level (GPD).

Efficiency is important in the production of clean water. Some units can produce 1 gallon of filtered water for every 2 gallons of wastewater, while others produce about 20 gallons of wastewater for every 1 gallon of filtered water.

The efficiency of clean water produced depends on efficiency, temperature, and pressure so it’s not purely related to the overall quality of the system.

Wastewater from your RO DI system can be used in your garden, to wash your car, and or to wash dishes.

Brand reputation

RO DI systems are a critical part of your reef tank set-up. You will want to purchase a system from a company that has been in the water filtration industry for a long time, with products well-known for efficiency and reliability. This is not the place to take shortcuts.

Ease of installation

If you’re brand new to the world of reef tanks, pick a RO DI system with easy installation. Simple and clear installation instructions are ideal for all levels of aquarists.


Your RO DI system will require regular maintenance, such as cleaning the membrane and canister. The easier your model is to clean, the more likely you will be to stick to a maintenance schedule. Don’t underestimate the value of easy maintenance.

What else is included?

Some models are fully-loaded and contain all of the necessary components. Others are bare-bones and will require you to purchase some key pieces of equipment.

For newbies, we absolutely recommend purchasing a model that has everything included. Examples of additional pieces of equipment that could be included are a faucet adapter, a tank that collects and stores the purified water, and a pressure pump that speeds up filtration.

Customer support

Customer support is extremely helpful if you need any technical assistance. Maybe a part breaks under warranty or your system needs repairs. Customer support is your ticket to a good experience.

Also, we recommend paying attention to the warranty provided with your system. It is peace of mind to have longer warranties.


Read the reviews on the systems you’re considering purchasing. They can be a goldmine for information. While we recognize that not all customers give accurate feedback, it is an opportunity to look for trends. Don’t over-index on one particular review but instead, read a variety of reviews, both recent and older.

Can You Just Purchase a Cheap RO DI System?

RO DI Systems can be pretty pricey. If you’re purchasing one during the set-up of your tank, maybe you want to avoid spending an additional several hundred dollars. Money is not unlimited so you’re wondering if purchasing a cheaper system could be a way to save some money.

First, the good news. There are less expensive options even when it comes to an RO DI system.

Now, the bad news. However, the trade-off is that the cheaper models usually lack some features and filtration stages. Or, the material used in the construction is cheaper, making it more likely to break down over time.

So, yes, you can purchase a cheaper model. However, it will likely lack certain conveniences and may end up costing you more in the long run if you need to replace it.

How Much Water Pressure Should A RO DI System Have?

Membranes in RO DI systems love water pressure and perform better in high-pressure situations. For example, Filmtec membranes have a max pressure rating of 300 psi. However, this doesn’t mean that all of the other components of the RO DI systems can handle this level of pressure.

Residential plumbing code typically calls for no more than 80 psi line pressure.

Some aquarists use booster pumps in order to elevate the pressure from their residential line. If you use a booster, plumb it in after the prefilter housings to avoid exposing them to the elevated pressure (they’re often not able to handle higher pressure).

Maintaining an RO DI System

As with any other filtration system, it is important to do some basic maintenance tasks to keep your RO DI system working well. In terms of cleaning the membrane and canister, the frequency may depend on how much water you generate.

Sediment Prefilter

The sediment pre-filter cartridge protects the system from clogging by blocking physical particles of iron oxide and sand. The pre-filter will eventually clog with sediment and become coated with slime. Besides being able to see the build-up, the pressure gauge readings will also decline. If you see the pressure changes, go ahead and replace the pre-filter. You should also plan to replace it every six months.

Carbon Cartridges

The carbon cartridge is the next step of the filtration process after the prefilter. The carbon cartridge removes chlorine and chloramine. It can also remove pesticides. Over time, this filter will get coated with slime. We recommend changing your carbon filters every six months.

Reverse Osmosis Membranes

The reverse osmosis membrane, also known as TFC membrane, is a semi-permeable membrane. The system forces water through the membrane, removing minerals, nitrate, phosphate, and heavy metals. The membrane functions by catching the contaminants from the water and flushing them away.

To maintain this system, you will need a flush kit. The flush kit will loosen and wash away minerals and contaminants that clog the membrane. We recommend flushing your membrane a few times a year.

To monitor the overall status of the RO membrane, use your TDS meter. When TDS levels consistently rise over time, this signals that the RO membrane needs to be replaced.

Deionization Cartridge

A deionization cartridge is the last stage. DI uses ion resins to bond with silicates, phosphates, and heavy metals. When the ion resin is depleted, deionization stops occurring. Because of this, you need to stay on top of your cartridge changes so your tank parameters stay stable.

Some DI cartridges use color-changing resin to indicate the ideal changing time. We also recommend regularly testing the water with a hand-held TDS meter to monitor water quality.

Best RO DI System for Reef Tank Options

APEC Water Systems Ultimate RO-Hi Water Filter System, 90 GPD

This is a great option if you have a large reef tank. APEC is a US-based manufacturer and one of the major brands in the water filtration industry. Their products come with a two-year warranty with excellent customer support so you know you’re making a good purchase with this option.

The APEC RO-90 is a 5-stage, quiet, reverse osmosis water system with no deionization. It is a quiet system. It is capable of removing up to 99% of water contaminants including chlorine, iron, and lead.

Please note that this system does not have a deionization stage. However, it is still powerful enough with the 5-stage RO filtration to produce reef safe water.

This system can produce 90 gallons of clean water daily, assuming your water pressure is at least 60 psi. Most aquarists don’t necessarily need 90 gallons daily, so this is why this option is best for large tanks.

This system is easy to install, with most users being able to get the system up and running in about two hours.


  • Can produce 90 gallons of clean water per day
  • No deionization stage


  • Effecient at removing nitrates from water
  • 2-year warranty
  • Quiet operation


  • Membrane filter can be difficult to open

AquaFX Barracuda RO/DI Aquarium Filter, 50 GPD

AquaFX is a well-known manufacturer of RO DI systems. They produce reputable, reliable RO DI systems. This option has a simple construction and the ability to output 50 gallons per day.

This system has 2-micron chlorine carbon block filter that is very effective. It also has a high flow and high rejection TFC membrane.

The unit does not come with a TDS meter so plan to purchase one so you can monitor the output over time. We recommend purchasing a flush kit to make sure the RO membrane is operating at peak condition. Also, RO membranes can be pricey to replace so it makes sense to try to keep it in good condition.


  • Produces up to 50 gallons of clean water per day
  • Has deionization stage


  • Reputable brand
  • Helpful customer service


  • Flush kit not included

AQUATICLIFE Aquatic Life RO Buddie Plus DI Four-Stage Reverse Osmosis Deionization Unit 50 GPD

The AQUATICLIFE Twist-in RO DI Water System is a great option for small tanks or aquarists without a lot of extra space. It is easy to install and uses cartridge-changing technology to make the maintenance process a snap. We find the cartridge changing process to be the simplest maintenance option on this list.

This is a great option for beginner aquarists who are new to RO DI water and don’t want to break the bank on a unit. It comes in different sized units so you can get the water output you need. However, some users cautioned that their units underproduced relative to their stated output, so it might make sense to slightly size up.


  • Multiple GPD options
  • Unique cartridge changing technology


  • Extremely easy maintenance
  • Affordable


  • Less output than stated
  • Might need to purchase booster pump to increase pressure and GPD

LiquaGen – 6 Stage Portable Dual Use Aquarium Reef/Deionization 

The LiquaGen RO DI system for water filtration is great value. It is an affordable way to create super clean water for your reef tank. It is also extremely easy to install because it comes pre-assembled and ready for installation. This means the whole installation process should take you about 5 minutes.

LiquiGen’s RO DI system is not the most efficient option if you have extremely hard water. It is designed to only handle water that contains less than 350 ppm of TDS. Some municipalities have water that contains around 700 ppm of TDS. If that is your situation, LiquaGen would not be the best choice. If your water is less than 350 ppm of TDS, LiquaGen guarantees clean water with 0 ppm of TDS.

This option comes with a 2-year warranty and lifetime customer support.


  • Comes pre-assembled
  • 6-stage filtration


  • Affordable
  • Easy to install
  • Can produce water that’s completely purified (0 TDS)


  • Will not produce 0 TDS water if the incoming water contrains more than 350 ppm TDS

SpectraPure MaxCap RO/DI System

Big reef tank owners (or heavily stocked tank owners), this option is for you. The SpectraPure 5 Stage 180GPD MaxCap is the top-of-the-line RO DI filtration option. This system comes with not one, but two 99% rejection rate RO membranes. It also includes pressure gauges, dual TDS meters, and an upgraded DI resin.

This unit is extremely efficient with a 2:1 waste ratio rate. It is the most efficient unit on this list. This means you will save on your water bill in the long term, which can help offset the larger investment cost of this system.


  • 2:1 waste water ratio
  • Produces up to 180 gallons of clean water per day


  • Extremely effecient
  • Excellent filtration via two 99% rejection rate RO membranes


  • More expensive than other options

Fountainhead RO/DI Reverse Osmosis Aquarium/Reef System, 75 GPD

Fountainhead is a USA-based manufacturer that produces water filtration systems for the home, so this unit is not aquarium specific. The Fountainhead Water Systems RO/DI System is a comprehensive unit that offers a 6 stage filtration for an affordable price!

This unit does not come with a pressure valve or TDS meter so you will need to purchase both of these elements. Additionally, this unit can be a little confusing to install so we do not recommend this option if you’re new to RO DI systems. Instructions are noted to be lacking by multiple users. If you know what you’re doing though, this system is an affordable way to get to 0 TDS for your reef tank.


  • 6 stage system with universal RODI cartridges
  • Produces up to 75 gallons of clean water per day


  • Affordable


  • Difficult installation
  • No Pressure Valve Included
  • No TDS Meter included

iSpring RCC7AK, NSF Certified 75 GPD, 6-Stage Reverse Osmosis System, pH+ Alkaline Remineralization 

This iSpring Reverse Osmosis filtration system is extremely easy to install. It comes with a comprehensive instruction manual and links to online videos that walk you through the process, step-by-step. They also have great customer support so you know you will be able to install this system without a ton of problems.

This is a reverse osmosis only option and it does not include a DI cartridge. It has 5 stages of filtration that does a great job at producing clean water that is safe to use in your reef tank.

One unique feature with this iSpring model is the sixth unit called the Alkaline Remineralization filter. The focus of this cartridge is helping the water recover the essential minerals lost with the harmful compounds during the filtration process. It helps the water be slightly less acidic.


  • Contains unique remineralization filter
  • 5 stages of filtration


  • Easy to install
  • Lifetime customer support
  • Produces high-quality water for reef tanks


  •  Process can be slow at 75 GPD

Home Master TMHP HydroPerfection Undersink Reverse Osmosis Water Filter System , White

The Home Master Water Purification System is the finest water purification system for your reef tank. There are 9 stages of water decontamination in this device, from filtering to purification to UV sterilization. It is well-known for removing iron, which can wreck havoc on coral reef tanks. It is also known for effectively removing high nitrate content.

It is an extremely efficient unit, with the ratio of filtered water to wastewater at 1:1. This means you will have lower water bills over time than with some of the cheaper systems on this list.

Despite being such a powerful system, installation is pretty straightforward and does not require a plumber. To make installation easy, the tubing is color-coded, the instructions are clear, and you get customer support with a customer service agent who can answer questions as you’re installing.

The filtration process in this unit goes through 9 stages to eliminate 98% of contaminants, including chlorine, sulfide, chloramines, and fluoride. The water is also treated for pathogens with UV light technology.

The downside of this option is that it is expensive compared to other units on the list. However, we love the UV sterilization option and water effeciency and consider it worth the cost!


  • 9-stage filtration
  • UV sterilization capability


  • 5-year warranty
  • Durable BPA-free plastic construction


  • Can be noisy
  • Expensive

Best Ro Di System for Reef Tank Options: Conclusion

We hope you’ve learned a lot about RO DI systems and are now ready to purchase the best system for your tank. Have questions? Reach out to us! Our team of experts is happy to help guide you through the process–just contact us with any comments or concerns. Which model did you end up selecting, and why? What has been your experience with it so far? Let’s hear from you in our comment section below!

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