What is RO “reverse osmosis?” 
Reverse osmosis is a water purification process that uses various membranes to extract unwanted chemicals and particles in the water which could be harmful to aquarium fish. The particles and waste products are left on one side of the membrane and the pure water is left on the other – ready to use.

Why is RO important for the fish tank?
This purified water is ideal to use for marine tanks especially. As tap water can contain levels of nitrates, phosphates and silicates which can cause unwanted algae growths and blooms in the tank and have also been known to kill off sensitive corals – so for marine fish keepers RO and salted RO are the way to go.
It is also important to remember that this water will lack important minerals and chemicals that may be detrimental to the fish’s health and wellbeing as well as important for coral or plant growth. So it is always necessary to dose the water with any vitamins or minerals that are needed for the correct growth and fish in your tank. Also some freshwater tanks need special care to keep their PH levels stable, fish such as cichlids have more specialised requirements when it comes to water PH, so when using RO water make sure you have a good re-mineralising buffer as this will be beneficial.