In fact, dead plants can have a strong effect on pH. In aquariums, dead plants can rot, releasing ammonia and other nitrogen compounds into the water, thus reducing the pH. Although ammonia is toxic to aquarium fish, a decreasing pH will actually make it marginally less toxic
The ecological optimum represents the value of the factors within the ecological valence at which the life processes of a species take place best. The upper and lower limits of an ecological valence are its maximum and minimum. Beyond these limits, physiological processes are interrupted and death occurs.
