The pH is the count of free hydrogen and hydroxyl ions in an aqueous solution. Solutions with a pH less than 7 are taken as less acidic and with a pH level greater than 7 are basic or alkaline. The normal level of pH for pure water is 7 at 25°C. Plants do best in the pH range of approximately 6.0 to 7.0. But, some plants may prefer more acidic or alkaline conditions, such as blueberries (4.0 to 6.0) or hyacinth (6.5 to 7.5). The pH range of 6.5-9 is best for most of the fish. In aquariums, the water is supplied with bicarbonate and carbonate ions to prevent pH changes.