The pH level of tap water may vary greatly depending on the location and season. It is usually neutral at pH7.0, but it can become more acidic (approximately pH6.0) or alkaline (approximately pH8.0). Most of the aquatic plants used in the basic types of layout grow well in neutral to slightly acidic water and tend to show difficulties when growing in alkaline water. This is because carbonic acid present in water is converted into another form according to pH.

In alkaline water, carbonate ion is dominant and only limited species of aquatic plants can perform photosynthesis in this environment; whereas in acidic water, free carbon dioxide becomes dominant and provides the environment that is ideal for most aquatic plants to perform photosynthesis in. This carbonate ion is also the main contributing factor in the rise in pH of tap water. Carbonate ion is contained in lime stone and other rocks/stone in the form of carbonate (or calcium carbonate) and it, together with calcium ion, dissolves in rainwater on and under the ground.

Calcium ion causes the total hardness (TH) of water to rise while carbonate ion raises the carbonate hardness (KH); and based on this fact, it can be said that the tap water having a higher pH level (around pH 8.0) tends to contain a higher level of TH and KH. Some stones used as a layout material contain calcium carbonate and can contribute to the rise in pH level of tank water. When there is a problem of high pH level of tap water or increased pH level of tank water, the application of the additive “water softener” or mixing of RO water can be an effective solution. By simply adding to the tap water in a pail, the additive “water softener” or mixing of RO water lowers the pH of tap water and also decreases the KH by the reaction of acid and carbonate ion.

This additive is safe for aquatic plants and fish, since carbonate ion will be converted to free carbon dioxide (C02) by reaction with acid and the additive “water softener” or mixing of RO water will eventually be broken down into water and carbon dioxide. How much the pH level can be lowered by the use of “water softener” or mixing of RO water varies depending on the tap water quality, so it is advised to add “water softener” or mixing of RO water slowly while measuring the pH level of the water using the pH Kit.

Please note that TH does not decrease by adding “water softener” to the water. An effective way to decrease TH level is to use a water purifier with a demineralizing function (for tap water having a high TH level) and to install a Softenizer in the tank (when TH rises due to the stone used in the aquarium).