RO water is an abbreviation for Reverse Osmosis water. This is water that has been squeezed through an ultra-fine sieve to remove impurities. It can filter out particles so small that bacteria and dissolved minerals are stripped from the water.
Typically the water is pushed through a number of filters of different types. Most systems have the following filters:
1/ A sediment filter to remove waterborne particles down to 5 microns in size (that's 5 millionths of a meter or 0.005mm diameter). this is generally the most coarse filter used in a system like this!
2/ One or two activated carbon filters to remove chemicals (particularly chlorine which would ruin the RO membrane) and further polish the water.
3/ The reverse osmosis membrane. This is the heart of the filter and is a membrane that will allow water to pass through, under pressure, whilst stopping the mineral content from getting through. The pressure required is usually that of the domestic water system. The problem with these filters is that for every litre of water that is produced about 3 litres are wasted as the membrane is very in-efficient. The wasted water goes straight to waste or can be collected to water plants etc. The membranes are rated for the number of gallons of RO water it will filter per day, although these ratings are estimates and vary with temperature and pressure. A 100 gallon per day filter will produce about 25 gallons of RO water (based on the 1:3 RO:waste ratio).
4/ High end filter systems may have a deioniser filter after the RO membrane. This further reduces the impurity level in the water.
5/ High end systems may also have another activated carbon filter to finally polish the water.
These systems can remove over 99% of impurities in the water. The end result is super soft, pure water.
Unfortunately this water is SO pure that it is virtually useless to fish keepers. It needs to be re-mineralised. The RO system cannot tell the difference between good and bad impurities and just filters everything out. Without remineralisation the water will have a very unstable pH due to no buffering capacity. Remineralisation salts can be bought and added to the water to reintroduce beneficial minerals. The fresh RO water must also be reoxygenated as the RO process also removes oxygen from the water (it's THAT good!). Some people simply mix certain quantities of conditioned tap water into the RO water to remineralise.
This may seem rather long winded but there is a very important note to be added.
RO water need only be used when keeping fish that must have very soft water for some reason or another. The vast majority of community tropical fish do not need this as they have been conditioned to live in water of moderate hardness, or are capable of this anyway.
The most well known use of RO water is in the breeding of Discus which require very soft high quality water to breed successfully. RO water is also used in marine systems as the base for the salt water required (the salt is added to the RO water to the correct level of salinity).
So long as your fish are not required to be kept in very soft water (read up on them) you do not need RO water. If your fish are happy, leave it alone! It's better to have stable water parameters rather than an unstable perfect setup. Fish need consistency!
Hope that helps coz my fingers are bleeding now!
WK
