If you have a small tank it may be beneficial to purchase your RO water from your local fish shop as it costs around £2.50-£3 per tub. However if you have a large tank it will be a wise investment to purchase your own unit as over time the little £2.50’s could soon mount up.  Always remember to do regular tests on your own RO water as the membrane may need replacing without you realising and it won’t be RO water that you are adding to your tank.

With this in mind it is also wise to test the water you purchase from shops as it has been known that they haven’t changed the membrane on their units either which can have severe effects on your livestock.
RO water is more important in your marine setups if you are keeping or plan on keeping corals and anemones. All tap waters consist of different quantities of minerals depending on the area you live in so it is advised that you don’t take the risk of not using RO water.