Reverse Osmosis (RO)
Aquarium Resources

Reverse Osmosis (RO)

Reverse osmosis is a way of making the water really pure and bring the hardness down to non detectable levels. RO water is for instance used by aquarists who want to breed Apistogramma cichlids, since these fishes normally need really pure and soft water to reproduce successfully.

Before you start tinkering with the water chemistry in your aquarium, there are several things to keep in mind. To begin with, many fish species hate rapid changes. A rapid change from poor water conditions to ideal water conditions can actually be more dangerous than keeping the fish in poor water conditions. There is however plenty of exceptions to this rule and a major water change that dramatically changes the conditions in the aquarium is known to induce breeding in many species. Before you make any changes in your aquarium you should therefore research your particular species to find out if they need a slow and gradual change or if they can cope with dramatic alterations in water chemistry.

Secondly, mimicking the natural conditions of a fish is not always a good idea. If you purchase specimens that have been bred and raised for many generations in your area, they may have adjusted to the local tap water and will happily live, spawn and raise their young in such conditions. Fiddling with the water chemistry will then be a waste of your time, money and energy, and it can even be dangerous for your fish.

Thirdly, fish should never be kept in pure RO water. You can read more about this further down in this article.  

What is osmosis and what is reverse osmosis?

When two water volumes (or other solvents) are separated by a semi permeable membrane that blocks the transport of solutes, water will flow from the side of low solute concentration to the side of high solute concentration. If you apply external pressure on the side of higher concentration, you can stop the flow. If you apply even stronger external pressure, the flow will be reversed – i.e. you will have created reverse osmosis.

Getting an RO unit

Before getting an RO unit, ask yourself how many aquariums you plan to keep simultaneously, how big they are and how often you will change the water. If you for instance have two 100 gallon aquariums that you change 50% of the water in once a week, a big 50 gallon a day RO unit will be a waste of money. If you on the other hand believe that you will keep getting more and more aquariums that need RO water (many aquarists do), then getting a major RO unit from the beginning may save you money in the long run.

Never use pure RO water

As mentioned above, fish should never be kept in pure RO water with a 0 ppm of total dissolved solids (TDS) since such extremely pure water is not what they are used to from the wild. You can purchase chemicals from your local fish store and use them to add all the necessary ingredients that will make the RO water suitable for fish, but this can be quite expensive, especially over time. A cheaper way of turning pure RO water into usable RO water is simply to add some of your normal tap water or the very hard “waste water” that will be produced by the RO unit. Use your test kit to find out the ideal blend.

Adjusting the pH-value

Many fish species appreciates and addition of peat moss to the RO water since it will keep the pH-value around 5.5 and make the water tea colored – just as in their native habitat. You can either filter the water through the peat moss or let peat moss rest in a bucket of RO water for a week or two before using the water.


Didn't find the info you were looking for? Ask your question in our Aquarium forum !
Our knowledgeable staff usually responds to any question within 24 hours

Other Aquarium Articles

Aquarium Cycling - A guide on how to cycle your aquarium.
Aquarium Lighting - Learn the basics of aquarium lighting
Breeding Fish - Overview of How To Breed Tropical Fish
Beginner fish - Great Beginner Fish for the Beginner Aquarist
Bad beginner fish - Horrible Beginner Fish for the Beginner Aquarist
The beginners tank - A guide to your fist aquarium
Cleaning your filter - Basic filter management
Carbon and aquariums - Information on the effects of using carbon in your aquarium.
Cycling - A guide to aquarium cycling and how it works.
Don’t trust the staff in the fish store! - A text about why you shouldn't alway trust your fish store.
Dry and Rainy Seasons in the Tank - A detailed account of how to simulate seasons in aquariums
Electrical Safety and Aquariums - How to prevent electrical accidents.
Filtration pro and cons - thoughts about filtration
Fry Growth rate - Thoughts on how to optimize growth in fry.
Fish keeping for beginners - An introduction for beginners.
Aquarium Filtration- Llists and describes the different methods of aquarium filtration, and then discusses their benefits and drawbacks.
Finding the Triggers - Information about how to find breeding triggers in different fihs species.
Fry Savers - Information about fry savers and how to construct fry savers.
How to Buy Healthy Fish - A gew tips on choosing healthy fish.
Making a Pebble Cave - A guide on how to make your own pebble cave for your fish.
Modifying Water Chemistry - A technical article about modifying hardness, alkalinity and pH.
Optimizing Growth of Fishes - The factors affecting fish growth - some may be manipulated, others not.
Setting up a tropical freshwater aquarium - Detailed guide on how to setup a freshwater aquarium..
Reverse Osmosis and aquariums - RO is a way to get very pure water in which fish such as dicus and neon tetras can breed and raise their fry.
Safe Rock and Wood Aquascaping - How to make sure that the materials you use in your aquascaping are aquarium safe.
Selecting Community Tank Fish - A guide that will help you choose the right fish for you based on tank size and experience level.
Setting Up A New Aquarium - Help beginners setup their first aquarium
Slime coat - Protecting your fishes vital slime coat
Snails in Aquariums - The biology of snails. Their advantages and disadvantages in an aquarium. How to keep them in check.
Amazon Biotope Aquarium - How to set-up, stock and maintain an aquarium that replicates the Amazon watershed.
Wall mounted aquariums - Information about Wall mounted aquariums
Water chemistry in the aquarium - Hardness, alkalinity and pH are discussed technically, from a chemistry point of view.
Water Chillers - Introduction to water chillers.