Re: How do plants alter the pH of a fish tank?

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