Tap water is designed for human consumption and contains various salts, chemicals and minerals including nitrate and phosphate that ideally need to be removed for use within an aquarium system.

Reverse osmosis water is water with most of the salts and elements removed.

Within an RO Unit there is a membrane through which water is forced under pressure. The membrane will not allow the salts or any other water contaminants through and will actively separate the salts and contaminants from the water.

These salts will be concentrated and flushed away through the waste pipe allowing only high quality salt free water through the membrane for collection into a vessel for later use.

Many modern aquarium salts are designed specifically for use with RO water as the levels of calcium within the salt are high (420-450mg/l). Normal tap water can have a calcium level of up to 60 mg/l in some areas and so if it is used with this high calcium salt there is a risk of oversaturation and the calcium coming out of solution.