RO Water in Freshwater Aquarium

Published on 26 February 2024 at 23:49

Using RO Water in Freshwater Aquarium

RO (Reverse Osmosis) water has been used for a long time in Marine Fishkeeping, but in recent times has also become increasingly popular among fresh water tropical fish enthusiasts. This article addresses some of the basic questions about the use of reverse osmosis water. It is not an in depth study, but intended only as a guide to the simpler aspects so further study and research will be needed for specific applications.


What is Reverse Osmosis?

Reverse Osmosis is not a thing, but a process. In a nutshell, osmosis refers to the process whereby a strong solution becomes more dilute when molecules of a weaker solution pass into it through a semi-permeable membrane.

Essentially a semi-permeable membrane will allow water molecules to pass through, but its "holes" are too small to allow the solids through. The stronger solution creates "osmotic pressure" which is only relieved when the solution is at the same strength on both sides of the membrane.

Reverse osmosis works the other way, in reverse. Water is forced through a semi-permeable membrane under pressure and the contaminating, larger particles are rejected so we end up with essentially pure water which we collect and a strong solution of contaminated waste water.


Why is RO water used by Fishkeepers?

There are many reasons why some fishkeepers choose to use RO water. When tap water is filtered by reverse osmosis many undesirable elements are removed from it, or greatly reduced in concentration. The resulting product water contains very low amounts of dissolved solids and is close to neutral in pH with virtually no hardness or alkalinity. This makes it possible to provide a lower level of hardness or pH for fish, even where these levels are high in the original tap water. It also deals with excessive levels of nitrate, phosphate, heavy metals and chlorine in the water.


Do I Need to Use RO water?

The answer to this question in many cases is "No". Our tap water (in the UK), is usually perfectly adequate for fish-keeping as long as a suitable water-conditioner is used to neutralise chlorine and chloramines and reduce the effects of heavy metals.

Using RO water can be expensive and inconvenient and once started needs to be continued in order to maintain a stable environment in the aquarium, so it should not be started upon lightly.

Each individual needs to make their own decision about whether or not to use RO water, but for fishkeepers with one community tank whose tap water supply is good quality, low in nitrates and other contaminants, it will almost always be unnecessary, especially if fish are chosen to suit existing tap water levels of hardness and pH.

If the only problem is high nitrate levels in the water supply, it may be more economical and less trouble to simply use a nitrate filter rather than RO water. Similarly if the problem is the need to increase pH and hardness that can be achieved by modifying existing tap water and an RO filter is not needed.

RO really becomes useful where there is a need to lower pH and/or hardness, where multiple tanks requiring different conditions are kept and where water is badly contaminated.


Using RO Water

If you have decided you need to use RO water the first thing to be aware of is that it cannot be used "neat". Remember, the water now has no hardness and no alkalinity. It also has none of the essential trace elements needed for fish to be healthy and thrive.

All fish need some degree of hardness in the water, and some degree alkalinity is necessary to prevent pH swings. This means that some minerals and other elements have to be put back into the RO water to make it habitable.


Mixing RO Water with Tap Water

The simplest way to use RO water for softening water and lowering pH is to mix a proportion of tap water and RO together to achieve the desired levels. Reducing gH in this way works very simply. If you mix half tap water and half RO water the resulting gH will be half of the original value. So if the gH of the tap water is 20 and what is needed for the fish tank is a gH of 5, mixing RO water with  tap water will produce the desired result.

Achieving a lower pH value by mixing tap water and RO is a little more complicated because the kH value also needs to be considered and because pH values are logarithmic a mix of half RO to tap water will not halve the pH. In fact if alkalinity (kH) is high enough, pH may remain at the same level as the tap water.

Trial and error with different mix ratios and repeated testing is probably the simplest way forward with this, but dont add any of your mixes to the tank until you are certain you have a stable pH level.


Using 100% RO Water and Remineralising

This method is basically just as the title suggests. Starting with pure RO water, essential salts and minerals are added in proportions that create water with the correct parameters for the fish it is to be used for.

This needs to be done carefully and although minerals and compounds can be custom mixed by the individual fishkeeper, the simplest and most accurate way to reconstitute RO water is by using commercially available products.

There are many remineralisation products available, some of which are specifically designed for certain types of fish. These include mixes for Discus, general community tank mixes and many other variants. The choice of product is very much a personal thing for each individual. Trace elements can also be added in a similar way, with different formulae available for each situation.

As well as setting the required level of hardness (gH), alkalinity needs to be adjusted to a level which will maintain pH, and pH itself set. Some products cover all bases, with a liquid or powder to mix in proportions directed by the manufacturer to achieve a set of parameters.

Minerals for adjusting gH are generally added first, thoroughly mixed by adding a small powerhead or an airstone and pump to the container and running for 24 hours. A heater set to the desired temperature for the tank can be added too. Next, alkalinity is adjusted to the desired kH level and finally pH is set, just before adding the water to the tank.

In my case I use Seachem products for this whole process: Equilibrium for gH and minerals, Alkaline Buffer for kH (alkalinity) and Acid Buffer to balance pH.


Vital Points When Switching to RO Water

When you first switch to using RO to adjust hardness and/or pH in an established tank you MUST do this gradually over a number of partial water changes. Rapid or large changes in hardness or pH are very harmful and even fatal to fish, and that applies even when youre actually adjusting to give them their preferred conditions. Remember, at the moment they are used to whatever water they have, and although they will benefit from the change it must not be too rapid as they will need time to gradually adjust.

Once the change has been made you will need to continue with your new RO mix at EVERY water change. Suddenly going back to straight tap water is not an option. You will need to ensure you always have a supply of RO water and any minerals and adjusters to hand.

Your RO water mix must be consistent. You need to experiment and make notes so that you know exactly how to mix your water to achieve the same gH, kH and pH every time. Dont start adding your mix to the tank until you can be sure of being consistent.


Other Considerations

If you decide to start using RO, remember that it can be costly, time-consuming and requires a commitment to accuracy and to continued use. For those reasons, it may be wiser not to do it unless you really need to.

You will need to decide whether to buy your RO water from your LFS or whether to buy an RO unit to make your own. Both have drawbacks and benefits. Only you can really choose which option suits you best.

If you choose to make your own RO, bear in mind that as well as the cost of the unit itself you will need a TDS meter to monitor the levels of dissolved solids in both your tap water and the RO produced by your filter. You will need suitable food-safe storage containers and the space to store them, and there are other associated costs. Filter elements and the RO membrane need regular replacement, you may need a plumber to fit the unit for you and the extra amount on your water bill can be high due to the amount of water rejected and sent down the drain to produce a relatively small quantity of RO. You will need to inform your water supplier of your intention to install an RO unit and may incur additional charges for water drainage and be required to have a meter installed if you do not already have one.



RO water is a useful addition to the fishkeepers box of water-keeping tools. But you may not need it, it can be expensive and it is time-consuming. Think carefully before plunging in and I hope this article has helped you with some of the basics of using RO water.