What is Reverse Osmosis (RO) Water? Why Would I Use RO?

What is Reverse Osmosis Water?

Reverse Osmosis water (RO) is made by passing tap water through a semi-permeable membrane which allows water molecules through, but prevents solids in the water from passing through; the solids are rejected. An RO filter has two outputs; one which produces pure RO water with virtually no solids and the other which is “waste” water containing all the rejected solids.

There are usually at least three stages of filtration: water first goes through a sediment filter which removes large particles, grit and so on. The second stage is a high-quality carbon filter which removes with metals such as copper and chemical contaminants such as chlorine, chloramine, pesticides and other unnecessary chemicals. The RO membrane itself is the third stage. The initial sediment filter and carbon filter stages are essential to prolong the life of the RO membrane and to prevent damage to the membrane which would make it far less effective.

RO-filter-diagram-fish-keeping-aquariumsMarine fish keepers add a fourth stage after the RO membrane which passes the RO water through deionising resin (DI) to take the total of dissolved solids (TDS) down to absolutely 0. This is not usually needed for fresh water tropical and cold water aquarium fish as the water produced by a 3 stage reverse osmosis unit is usually pure enough. Some do go even further and add additional stages with finer sedimentation media or extra carbon stages  before the water reaches the membrane. The diagram on the right shows how a three stage RO unit filters the water.

All stages of an RO filter need to be replaced regularly. The sediment and carbon stages should be replaced at least every six months and the RO membrane should be replaced when it starts to allow more solids through, which you can check with a TDS meter. If a DI stage is used, this needs regular replacement as it retains the solids it removes and eventually becomes saturated and can’t absorb any more.

A standard three stage unit will reduce the TDS of the supplied water by around 95% or more. When this drops to 90% or less it’s time to replace the membrane.

Why is RO water used by fish keepers?

When tap water is filtered by reverse osmosis many undesirable elements are removed or greatly reduced in concentration providing virtually pure water with very low amounts of dissolved solids and  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 fishkeeping as long fish are chosen that suit the hardness and pH of your supply and a suitable water-conditioner is used to neutralise chlorine, chloramines and heavy metals. There are some circumstances where using RO is essential or at least very useful:

  • Where your supply is too hard and/or too alkaline for the fish you want to keep
  • Where the supply has a very high nitrate level and you have fish that are intolerant of nitrate
  • Where you want to tailor water to a very precise level for breeding fish or for wild caught fish
  • If you keep multiple tanks, all needing different parameters
  • Where your water supply varies in its hardness/pH/nitrate level because your supplier frequently switches sources

If the only problem is high nitrates in your water supply, it may be more economical and less trouble to simply use a nitrate filter rather than RO water. Have a look at this article on nitrate control in the aquarium.

If the problem is that your water is too soft and acidic for the fish you want to keep you won’t need RO as pH and hardness can be raised very easily, but where they need reducing, RO is the only reliable and safe way to do it successfully.

Questions about this? Click here.

Has this page been helpful? Buy me a coffee 🙂

More on water chemistry and quality:


Bookmark the permalink.

Comments are closed.