Aquarium plants can alter aquarium pH, either raising or lowering the pH level under different circumstances. When pH lowers, water becomes more acidic. When it rises, water becomes more basic or alkaline. Plants change water pH through several mechanism.
Given enough light and carbon dioxide, plants absorb carbon dioxide and produce oxygen. Carbon dioxide will lower the pH of aquarium water, making it more acidic, while oxygen has no effect on water pH. Therefore, when plants absorb carbon dioxide, thus removing it from the water, the pH will rise. Typically, this has a small, but measurable effect on pH. You actually can measure water's carbon dioxide content indirectly by measuring changes in pH. Under normal aquarium conditions, this change is not enough to have an impact on fish, since carbon dioxide levels high enough to alter pH radically would probably be toxic to fish.
In addition to carbon dioxide, plants can metabolize several other chemicals and change pH. For example, plants can absorb ammonia and other nitrogen compounds. These compounds typically encourage biological reactions that produce free hydrogen ions, which lower the pH. By absorbing these chemicals, plants prevent the pH from dropping. Conversely, some plants, like Egeria, absorb minerals, like calcium, from the water. This reduces the buffering capacity of water, which can lead to a lower pH.
Dead plants actually can produce a strong effect on pH. Dead plants will rot in aquarium, which releases ammonia and other nitrogen compounds into the water, thus lowering the pH. While ammonia is toxic to aquarium fish, a dropping pH will actually make it slightly less toxic. This still is not an ideal situation for the aquarium fish, and should be addressed by removing dead plants and performing a water change as soon as possible.
pH and Plants
While aquarium plants can affect pH, pH also can affect aquarium plants. Like fish, plants typically are adapted to the water chemistry of their home waters. For example, Amazon plants, like the Amazon swordplant, prefer soft, acidic water. Conversely, Egeria has adapted to hard, alkaline water, and does best in aquariums that mimic this water parameter. Still other plants, like Anubias, occur over a wide area, and can adapt to a range of pH levels.
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