How to Use RODI Water to Create the Best Water for Your Aquarium

Water is the main ingredient of reefing and one of the most complicated choices you have.  The common options of RODI water (can be referred to as RO/DI), RO, de-iodized, bottled, Mineral, filtered, and tap water, and the debate between cost versus the benefits can be confusing.

Can I use purified water in my fish tank?  Purified water is better for your fish and plants.  It keeps your aquarium cleaner and preserves the life of your equipment too.

Osmosis between the fish and the water environment can kill your fish if the TDS levels are too high.  The water inside your fish and the tank will try to find equilibrium, and this will affect the blood pressure, digestion, and immune system of your fish.

The fish tank fish water removes TSS (Total Suspended Solids) plant material and fish waste but not the TDS (Total dissolved Solids) minerals and trace elements.

Many factors influence the TDS levels in your tank:

  • The natural nitrogen cycle of your fish tank
  • How much and what you feed your fish and plants
  • Anything you add to your aquarium

Adding extra TDS with water will cause a problem, and this is why it is a good idea to use a fish tank water purifier to clean water before you add it. This debate raises questions as to why you need to remove all the elements of water only to re-add them by remineralizing it for freshwater aquariums?  How many filters do you really need? Providing the best for our pets should be at the heart of all our decisions.

Below we discuss the different water options, how they are created, and the benefits and problems of each to help you make the correct choice for you and your fish tank inhabitants.  Some fish do not mind changes in the PH levels but may not breed, and others are delicate. 

Included are 7 of the best RODI systems for reef tanks available, including 2 portable systems if you do not want a dedicated water supply leading to the device. 

If you want to create a breeding tank, your fish need to be in an environment with the ideal conditions for that fish, and RODI water gives you more control over the elements. If you are interested in breeding, you may want to check out our article Everything That You Should Know About Koi Fish Breeding.

What is RODI Water? And Why is it Suitable For Aquariums?

RODI water is the best for your aquarium because it gives you control over the KH, GH and TDS levels inside the habitat. It keeps your fish tank cleaner and protects freshwater and saltwater aquarium filter systems.  Adding pure water with the correct concentration of minerals is easier and safer than stabilizing the elements after it is in your aquarium. 

Water contains different levels of dissolved salts, buffers, and nutrients.  The levels of these additives change four important elements within your aquarium.  RODI water removes all these elements and thus reduces the PH level of water. 

The 4 main elements of water include:

  • PH level for most fish needs to remain between PH 6.5 and 7.5.
  • KH (buffering) level is the waters’ ability to retain a good PH during the nitrate and nitrite cycle.  Test kits test the KH level of your water, and anything above 5 KH is acceptable.  The KH is affected by the element carbonate.
  • GH (hard or soft water) Level is affected by the amount of calcium and magnesium present in the water.
  • Salinity all TDS including salt (freshwater has salt too)

The approximate levels TDS elements in your fish tank:

  • Nitrites and nitrates less than 40 ppm
  • Chlorine 0 ppm.
  • GH (general hardness) depend on the fish you keep in your aquarium
  • KH for freshwater fish tank between 120 – 180 ppm
  • KH for Saltwater fish tank is between 180-300 ppm

Keeping the changes in these levels stable will keep your fishy friends happy and healthy. By adding seashells, coral, limestone, marble chips will increase the KH and GH of your fish to keep the atmosphere stable.

If you use a RODI unit as part of your freshwater filtration system, you need to remineralize it, but as part of a saltwater fish tank filter system, you do not need to.

RODI water keeps your fish tank cleaner, and this means less work for you.  Other options to keep your fish tank cleaner include adding plants.  For more on this read our article Using Aquarium Plants To Create a Living Artwork. Or creating a self-cleaning aquarium Self Cleaning Fish Tanks That Actually Work and you will only need top up water, and depending on your water requirements, one of the portable RODI units may be suitable for you.

What Is RODI Water?

RODI water stands for Reverse Osmosis and De-ionization. RODI units convert tap water into clean, purified water free of TDS. If it has a carbon pre filter, it will remove chlorine and chloramine compounds, too (most new machines today do).

RODI water gives Aquarists full control over the different elements in their fish tank, and this is a must for people who want to keep delicate fish or corals.

De-chlorinating Water the First Process When Creating RODI Water

This process of making RODI Water may sound complicated, but creating RODI water systems is easy as the RODI Unit takes care of this for you.  The first filter is a carbon filter that removes chlorine and chlorinates before it moves to the next filter.

Reverse Osmosis the Second Process When Creating RODI Water

Reverse Osmosis removes all organic materials and some inorganic elements from water.

Most of us understand Osmosis, where the water with a lower concentration of an element moves through a membrane to the solution with a higher level of that element to create equilibrium.  Reverse Osmosis stops this process by pressure and trapping elements on one side and collecting pure water on the other side of the RO membrane.

Water enters the system, causing pressure higher than that of osmotic pressure, thereby forcing water through the RO membrane with pores 0.001 micrometers that prevent microscopic particles from passing through. In comparison, other filtrations systems have membranes with pores the size of 0.01 micrometers.

De-ionization the Third Process When Creating RODI Water

The Deionized process removes inorganic material like irons (salts) from water. The DI cartridge has cation and anion resin that attract negative and positive ions replacing it with Hydrogen (H) and hydroxide (OH) elements creating H2O.

These are the primary filters in all RODI units, and the result is pure water that contains little to no trace elements.  Distilled water may be cleaner if it is double or, in some cases, tripled distilled as used by pharmaceutical companies, but it is expensive and complicated for home use.

The Difference Between RO and Deionized Water

A RODI unit uses both the reverse Osmosis and the deionized process to purify water.  Reverse Osmosis and deionized water are distinct types, each with its filtration system.  Many reefers have a reverse osmosis fish tank, but this is not ideal as it cannot remove the minerals from the water.

RO water aquariums don’t have many harmful elements tap water may contain, but reefers have less control over the TDS elements in these fish tanks.  Minerals affecting PH, GH, and KH are removed with a deionization filter system keeping your tank cleaner for longer. 

RO water freshwater aquariums have higher concentrations of salt, which can be unsuitable for some types of fish, and others will not breed if the levels of PH and salts are not within the ideal parameters.

The relationship between your aquatic life and RODI water is the perfect match as it keeps the TDS solids in check, making it better than an aquarium reverse osmosis system.

A RODI system for saltwater aquariums is vital; the minerals in tap water can change depending on the needs of the provider, making RO water for saltwater tanks unsuitable.

Both RO/DI systems can be bought separately, but it is easier to use as one RODI unit.

The Problems with RODI Water

You must maintain your RODI unit and test the water before you use it, and you need a good testing kit able to detect all trace elements.  The sediment builds up on your carbon filter, and the RO membranes will break over time. 

The RO membranes need to be replaced to protect your DI resin from being exhausted. Test your water; it is safe to use if the TDS is under 9 before replacing your DI cartridge.

The GPD measurements on RODI units is the amount of water it processes and not the amount of pure water it creates; the impurified water is discharged as wastewater.  The wastewater is safe to use in a swimming pool or garden to decrease the amount of water your household uses.

Periodically the system must be flushed out to clean and give it a longer life.

Different Types of Water and Why It Is Bad for Your Fish Tank

It is better to understand the different types of water you can use and the problems it can create before you use it in your aquarium.

Distilled Water

Distilled water is the softest and purest form of water, as it has been passed through a distillation process.  Water is boiled to create steam which is passed into another part of the system where the steam condenses into water, leaving most impurities and elements behind, it is expensive to buy and not a practical solution to create at home.

The fish tank habitat needs to remain stable, and any significant change can damage or even kill your fish.  Distilled water has little to no KH (buffering) ability making it unsuitable form of water for your aquarium as it causes large variations in the PH level of the fish tank.

Tap Water

The elements in the tap water vary depending on the area you live in, any repairs conducted on the system, and the pipes that run water to your home.  For the most part, only chlorine and chloramine (when chlorine is attached to another element) compounds are bad for your fish tank, but the other trace elements can cause problems.

The TDS range of tap water is between 300 – 600, and, by comparison, RODI water is almost 0, and that makes a difference. The TDS can include chlorine, chloramines, copper, nitrates, nitrites, phosphates, and silicates.

  • Phosphate levels higher than .2 mg/L stimulates algae growth
  • Any trace of copper will damage fish, corals, and plants

If you keep hardy fish, you can use tap water as long as it does not pass through copper pipes and dechlorinate it.  To remove chlorine, either leave tap water in a bucket overnight or use a water conditioner like the Seachem Prime Fresh and Saltwater Conditioner to make it safe to use in your fish tank.

Adding another layer of filtration is a good idea, and it is easy to create a tap water filter for fish tanks.  Any filtration system that removes elements like chlorine and does not add anything as you pour it out can be used as an aquarium tap water filter.  The Epic Pure Water Filter Pitcher.

Bottled Water

Bottled water may be a bit costly, but it is safe to use, test to see if the elements are within the required parameters before using it as brands differ.  Bottled water is treated by reverse osmosis or distillation, but many elements reintroduced to the water.

Mineral or Spring Water

Mineral or Spring water comes from underground streams and is not recommended as the elements in the bottle are changeable and may have the same TDS as tap water.  Mineral and spring water is expensive and adds no additional benefits than dechlorinated tap water.

Filtered Water

The only difference between filtered water and tap water is that the water has passed through a carbon filter to remove chlorine.  It may have an additional micron filter, which will remove more impurities.  This water is safe to use in your fish tank and is inexpensive.

How do you make RODI Water?

Although explaining the process of RODI water is complicated, how you make it is easy.  Attach a RODI unit to a tap or water inlet pipe (depending on the machine you use).  The water passes through the filters and gives you RODI water.

Store the water in a food-grade quality container and place a lid on to keep it clean until you need to use it.  Periodically you will change the filter once they are discolored.  The cartridge needs to be replaced when the TDS kit produces a reading greater than 9; if your fish are delicate or you have a breeding tank, change the filter when the TDS level is between 4 – 6 TDS.

Also, read – How to Choose the Best Aquarium Driftwood

How to Flush the RODI System?

It is easier to do if you have a flush valve located on your wastewater line, turn it to the flow position, and leave it running for about 30 minutes.  If your system does not have a flush valve, remove the RO membrane from the wastewater line and run your system for thirty minutes.  After the system has been cleaned, reattached the RO membrane to the wastewater pipe. Do not forget to turn off the water supply to the system.

Different RODI Water Filtration Systems

RODI Units are measured in GPD, which means gallons per day.  Be careful when selecting your product, the temperature of the water and water pressure determine how many gallons per day your system can process.  The testing variables are based on industry standard and not average for households, and results differ considerably.

How RODI Units Filter Water

The RODI filter has three different filters a carbon RO and DI filters, each completing different processes inside the unit.

Carbon Filter

The best RODI system for reef tanks has more than one carbon filter to protect the RO membrane better and thereby gives it more life.  This filter removes chlorine and debris from the water and protects the RO membrane from being damaged.

RO Membrane

The Reverse Osmosis membrane filters removing 85 – 95% of particle matter from the water, including copper and lead.

DI Cartridge

The DI system uses a chemical exchange to remove salts and irons from the water as it passes through the DI filter that draws out up to 99% of the minerals.

Different systems have added filters that may include:

  • A UV filter that cleans microorganisms
  • Additional prefilters to remove more sediment particles before the water reaches the RO membrane
  • Additive cartridges that add calcium and magnesium salts into the water (this is not a good option for reefers as you lose control of the elements in your water)
  • Some units have 2 DI cartridges which may remove more minerals from the water
  • Additional filters after the DI cartridge to improve the odor and taste (not necessary for reefing)

Top 7 RODI Units

#1 AquaFX Barracuda RO/DI Aquarium Filter

AquaFX is a trusted brand that manufactures high-quality products.  This barracuda RODI unit has two options a 50 and 100 GPD.  2 pre carbon filters protect the RO membrane.  The best part is this system is easy to install and use.  The added pressure gauge keeps the system manufacturing water optimally.  It is efficient but produces more wastewater.


  • The added pressure gauge keeps the system efficient
  • The water is clean


  • The system is more expensive than other units
  • This RODI Unit produces more wastewater

Our Verdict:  This system may be more expensive, but for delicate reefs, this system is a must-have item.

#2 Koolermax AR-122 6-stage RO+DI Aquarium Reef Reverse Osmosis Water System

The Koolermax is an affordable 6-stage system and has a pressure gauge and indicator that notifies you when you need to replace some of the filters.  Depending on the water quality entering the system, the filters only need to be changed once a year.  The filters include 1 sediment, 2 carbon filters; this protects the RO membrane.  The water passes through 2 DI cartridges ensuring the water is pure.


  • A 6-stage filter system that protects the RO membrane
  • The filters are long-lasting
  • It is an affordable RODI unit


  • Reefers have complained about the installation
  • You may not need all these filters

Our Verdict:  This system is perfect if you are filtering well or dirty tap water.  If compared to the AquaFX Barracuda, you may want the additional carbon filter and for extra security the extra DI cartridge for extra peace of mind.

#3 Aquatic Life RO Buddie Three Stage Reverse Osmosis

The Aquatic Life RO Buddie is a three-stage system that is affordable and easy to set up and use.  This system has one prefilter, the RO membrane, and the DI cartridge.  The best part of this system is that it is compact, and if you are renting your home, it is easy to install and uninstall.


  • This RODI Unit is affordable
  • The unit is easy to uninstall
  • The Aquatic Life RO Buddie is compact and discreet


  • This system only has three filters
  • Without the pressure gauge, the GPD may be less than optimal

Our verdict:  If you are renting the ability to remove the system, makes this a good option for you.  Small and compact and removes enough particles for most reefers.

#4 LiquaGen – 5-Stage Reverse Osmosis and Deionization

The LiquaGen has 5-stages with 2 carbon filters that will protect the RO membrane.  This system is efficient, although slower than other RODI Units.  It is bulky, but if you have space, it is worth having it for the duel carbon filter.


  • This machine is an affordable 5-part filtration system
  • Easy to assemble
  • The unit has 2 carbon filters


  • This system is slower than others
  • The RODI unit is bulky

Our Verdict:  The duel carbon filter is fantastic, but the machine is bulky and less efficient, but you have peace of mind that the water is pure.  If you have space, the additional carbon filter is a great addition.

#5 SpectraPure MaxCap RO/DI System

The SpectraPure MaxCap is a 5-sage system including a sediment and a carbon filter before reaching the RO membrane and 2 DI cartridges. The additional 2 DI cartridges will remove all salts from the water, and this system is perfect for delicate reefs.


  • This system has 2 prefilters to protect your RO membrane
  • The added 2 Di cartridges give the water more time to complete the chemical extraction process


  • This system is pricey

Our Verdict:  This unit is more compact than some others, and the additional pre filters and additional DI cartridges make this an ideal unit for delicate reefs or impure water sources like wells.

What if your water requirements do not need a dedicated water supply into a RODI unit?  One option is turning off the water supply, but portable RODI units are another, which will decrease the amount of wastewater you create. Mobile systems are perfect for small aquariums.  For larger tanks with smart planning and several storage containers, you can use one of these too.

If you want to find the perfect small fish tank for you, read our article How to Find the Best 10 Gallon Fish Tank for your Home.

#6 Aquatic Life Twist-In 75 GPD

Even if you live in a dorm room, you can still provide the best water for your fishy friends.  The aquatic life twist is a popular portable system if you do not want to attach a dedicated water supply to a RODI unit.  More compact than the 0PPM Portable 150 GPD Reverse Osmosis RO+DI Filtration unit and more attractive too.


  • The Aquatic Life Twist is compact
  • This system is easy to set up and use


  • This process generates less water than traditional RODI units.

Our Verdict: This is an affordable compact solution to create RODI water where you do not have or need a dedicated water supply. Portable units save money and use less water overall.  This unit is compact and makes RODI water quickly.

#7 0PPM Portable 150 GPD Reverse Osmosis RO+DI Filtration

The 0PPM Portable system is affordable and portable.  All you have to do is attach the RODI unit to any faucet and turn it on, and you have a RODI water system, but you may have to purchase additional adaptors to suit the faucet you have.


  • Portable and compact to store away between uses
  • This RODI unit is affordable


  • It has limited GPD for large tanks
  • You may have to purchase the correct faucet adaptor
  • It takes more planning to make sure you have enough water on hand

Our Verdict:  If you cannot dedicate water supply to a RODI unit, this one is perfect for you.  Small and compact to store between uses, and once you have the correct faucet adaptor, it is easy to turn any faucet into a RODI water system.

How to Remineralizering RODI Water for Freshwater Aquariums

The first step in remineralizing your water is to understand the quality of the water inside your aquarium.  TDS levels fluctuate according to the feeding cycle of the plants and fish.  Test the different elements of your tank over a period to gauge how the different elements change.  Compare these changes over a period to the tolerance levels of your fish. 

To remineralize RODI Water add the correct amount of Seachem Equilibrium to the premade RODI water and mix well.  The amount of equilibrium you need will depend on what is in your aquarium and how hard the water needs to be, and this depends on the residents of your fish tank.  The required TDS in a planted tank needs to be higher as plants use the elements as food.

If you need to decrease, the TDS levels siphon 20% of your fish tank water, keeping 10% and discarding the rest.  Mix equal parts of RODI water and water from your fish tank.  Heat the water to the correct temperature and pour it into your fish tank.  Repeat twice or thrice a week until you achieve the correct TDS levels.

Remineralizing your water is only needed if you have a Freshwater tank.  For saltwater fish tanks, the marine salt you use to create saltwater will have all the minerals and salts your aquarium needs. 

ClassificationTDS PPM

If you are using RODI water in your aquarium, you only need a TDS meter to check the levels as all harmful elements have been removed.  You only have to use testing strips If the TDS levels spike by 10% to isolate the cause.  The culprit is usually one of the following: a missing element inside your tank, feeding schedule, or any substance you added to the fish tank.

FAQ’s on How to Use RODI Water

How Long Before You Need to Replace the Filters on Your RODI System? When you need to change the RODI filter depends on the capacity of RODI Water you make and the capacity of your machine.  As a rule, change your filter after you see discoloration, usually after making about 125 – 130 gallons of RODI water.  Adding additional chlorine filters will extend the life of your filter. 

For the DI cartridge testing your RODI water, if the TSD is higher than 9 TSD, it is time to change your DI Cartridge.

How to Remove Chloramines with a RODI System for Aquariums? Chloramine compounds (which are created when chlorine attaches itself to another element) are removed by the carbon filter on the RODI system.  Most units today have a carbon filter as part of the unit.

What Happens if I Run Hot Water Through My RODI System? The increased temperature does increase the amount of water it produces, but increasing the pressure is more manageable with better results.

Increasing the temperature of the water before it enters the RODI unit will increase the amount of water that flows through the system but also increases the number of TDS that can pass through by almost 40%, which is not desirable.  Another problem is the membrane itself is sensitive to higher temperatures, and reefers who have tried this have broken the RO membrane. 

How Long Can You Keep RODI Water? The length of time you can keep RODI water depends on the container used to store the water.  If it is light-proof with an airtight seal and does not release (leach) chemicals into the water, you can store if for 2 years.

Is RODI Water Good for Plants? Plants need calcium and magnesium, which are removed during the RODI process.  Only use RODI water if you feed your plants plenty of micronutrients or remineralize it, but rain and even tap water are better.

Is RODI Water Safe to Drink? A RODI unit does not sterilize the water from harmful pathogens, but If drinking water is passed through a RODI Unit, or if the unit has the additional UV light filter attached to it, then drinking RODI water will not harm you. 

Your body and teeth need elements like calcium and other minerals found in water, and making a long-term switch is not recommended by the World Health Organization (WHO).

Can You Use Reverse Osmosis Water in a Fish Tank?  If the TDS levels in your tap water are within the parameters of the needs of your fish, you can use RO water for your fish tank.  If the levels are not, you need to use a RODI water filter.  RO & DI units can be bought separately if you already have a device like a RO buddy all you need to add is a deionization filter system to create a RO DI water system.

Keep Your Aquarium Stable with RODI Water

Although reefers agree that RODI water is the best water for your fish tank and it will keep your aquarium cleaner, you do not have to use it.  Your need for RODI water depends on the residents of your fish tank and the changes they can tolerate.  If you have a delicate or breeding fish tank, RODI water is the best.  For freshwater fish tanks remineralize your water, and you are good to go.  The next best thing to RODI water is water created by reverse Osmosis, which has many of the same benefits. 

In the end, any water that has a high KH and does not contain chlorine, chloramines, and copper is safe to use in most aquariums.  If you decide not to use RODI or RO water, filtering or dechlorinating tap water is a good option.  Using this water will save you money and have many of the same disadvantages as the more expensive options of using bottled, Mineral, or spring water.

Also read – The Best Freshwater Aquarium Fish for a 55 Gallon Tank

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