|MadSci Network: Botany|
Plants can alter aquarium water pH in two main ways. Their production of carbon dioxide in the dark via cellular respiration will tend to lower pH because carbon dioxide can react with water to produce carbonic acid. In light, plant photosynthesis will consume carbon dioxide, which would cause the pH to rise. This would produce a daily shift in pH upward during the day and downward at night. Plants also absorb mineral nutrient ions, such as nitrate (NO3-), ammonium (NH4+), potassium (K+), phosphates (H2PO4-, HPO4--), calcium (Ca++) and magnesium (Mg++). If the plant absorbs more positive ions (cations) than negative ions (anions), then it excretes hydrogen ions (H+), which lower the pH. If the plant absorbs more anions than cations, it excretes hydroxyl ions (OH-), which raises the pH. Fish wastes contain lots of ammonium. The magnitude of plant-induced pH changes will vary depending on several other factors such as the amount of plant matter relative to the water volume, rate of plant growth, forced aeration, amount of light, initial water pH, buffering by dissolved substances such as phosphates, bicarbonates and carbonates or by calcium carbonate in shells, rocks or coral. Any pH changes caused by fish respiration, fish wastes, and microbes decomposing fish wastes would also have to be taken into account. There are quite a few webpages on aquarium pH. References Hershey, D.R. 1992. Plant nutrient solution pH changes. Journal of Biological Education 26:107-111. Aquarium pH websites
Try the links in the MadSci Library for more information on Botany.