What is RO Water?

If you’ve been reading a lot about setting up a saltwater aquarium, then you’re bound to have come across RO water.  Well, what is RO Water and do you really need it?  

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Tap water can be filtered through a Reverse Osmosis (RO) unit.  This filtering system will remove dissolved and suspended particles and produce purified water, also known as RO water.

The unit can easy be installed under a sink allowing you to make your own RO water rather than relying on anyone else to provide it for you.

However, RO water will not be as pure for saltwater aquariums without a DI canister.  What difference does that make?  Read on!

What's the difference between RO and RO/DI water?

The difference between RO and RO/DI water is simply that an RO Unit without a DI canister produces RO water and a unit with a DI canister produces RO/DI water.  

DI stands for De-Ionized.  This additional canister is important when running a saltwater aquarium because it removes any remaining phosphates, nitrates, silicates and other total dissolved solids (TDS) from the water that haven’t already been removed through the RO process.      

Is RO or RO/DI water necessary for a saltwater aquarium?

In most cases, saltwater systems would suffer from uncontrolled algae due to potentially elevated levels of nitrates, phosphates and harmful substances like copper if tap water was used.   It could also contain chlorine and chloramine which is toxic to fish and invertebrates.  This isn’t to say that all tap water contains these these but there is a high possibility, especially when living in a city, that it could do. 


You simply do not know what is in tap water 100% of the time and water suppliers may treat water with excess chlorine or phosphates at any time and you’d be none the wiser. 

This is why you it is advisable to at least use RO water in your saltwater aquarium.  However, while some may run a reef aquarium with just an RO unit the majority are in agreement that that they would not be running their tanks without using RO/DI water.  


Water quality and assurance.  Because if you know that your tank is being run with pure water then you know that it won’t be that contributing to any raised levels of phosphates, nitrates etc.  There are so many variables when running a saltwater aquarium that it’s never easy to blame one particular thing when things go wrong.  Using RO/DI water, you at least know it’s not the water you’re adding to your tank that is causing problems and can look at other causes.

Measuring water quality with a TDS Meter

What is a TDS Meter?

The pureness of RO water, or RO/DI water, is measured by reading the TDS (Total Dissolved Solids) with an inline TDS meter or a handheld TDS pen.  To read more on the differences between each one, you can read this article.)

The ideal reading for water for a saltwater aquarium is zero.  That means zero TDS (phosphates, nitrates, chlorine etc.) will be getting into your tank.

tds pen ro unit
inline tds meter ro unit

You can achieve this by having an RO/DI unit.  Without a DI canister included in the system, you will not achieve the purest water with a measurement of zero TDS but it is certainly better than using any other type of water such as tap water.  

Note that DI resin will need to be changed when the measured value starts creeping up as it becomes exhausted.  The frequency of this depends on the TDS of your tap water. The higher the TDS of your tap water, the more frequently you will need the change the DI resin.

Can you buy RO water?

Yes, RO water is available from grocery stores.  Most local fish stores will provide RO/DI water as they understand the need for pure water when running a saltwater tank.

If you are serious about keeping saltwater fish however, you might find it less expensive in the long run to invest in an RO/DI unit to install at home.  

Can you make RO water?

RO water is generally made using a Reverse Osmosis Unit which is connected to your mains water supply.

Take a look at these ideas on how to re-use RO/DI waste water.

My RO/DI Unit Setup

This is my RO/DI setup which is situated in the cupboard beneath the kitchen sink.  

The unit I use can process up to 50 gallons per day.  Most units are much of a muchness with the expense of them rising with additional features added.  

My unit is a standard unit that will produce 5 gallons in 2.5 – 3 hours depending on the temperature.  

You should bear in mind that should you live in a low water pressure area, you may need to invest in a booster pump.  RO units work by forcing the tap water through a semi-permeable membrane at pressure, therefore insufficient pressure will need to be addressed.

Most RO units are supplied with the parts required to plumb it into your existing pipework.  Using a stop valve, I turn the tap to run the unit and close it when done.

Most units also include a flush valve which helps maintain the pressure while running the water, and once done, you open the tap and flush the unit through for a couple of seconds.

This helps to clear it on any potential buildup on the membranes, thereby elongating the their life.

So there you have it!

RO units, in my opinion, are a must. Most local fish shops (LFS) have RO water for sale, but the expense of regular purchases will eventually exceed the expense of a one-off purchase which you will always have immediate access to.

Additionally, you may be lucky enough to have a LFS that constantly monitors their RO/DI water TDS, but more than likely it is in high demand and is very easy for their levels to exceed what you would like to put into your tank.

You will need to monitor your water produced regularly, and you will need to change your filters as and when required. But this is a simple process and potentially only needs doing twice a year, again depending on your tap water TDS.

The perceived technicality of the RO unit can be slightly intimidating if you are looking at pictures of them, but have never used one. I felt the same too, until I purchased my first unit and realised how very easy it was to connect up and use.

Hopefully this has explained a bit about RO Water for aquariums and how they may benefit you.

RO Water for Aquariums