Stenoion fish - fish that live in a specific or narrow range of changes in the acidity of the environment (trout, salmon, grayling, etc.).

In relation to fluctuations in pH environment fish are divided into steno- and euryionic. In the water of the sea pH varies little (7.5-8.5), marine fish belong to stenoionic. Fresh waters, unlike marine waters, are characterized by pH instability. This is caused by a variety of factors that direct the course of biochemical processes in the water body: the nature of soils of the bed and watershed, the chemical composition of the water source, photosynthetic activity of plants, especially in the period of ‛flowering' water, etc. As a result, sharp annual, seasonal and daily pH fluctuations are observed.

Stenoion species that prefer acidic waters (soils) are called acidophilic (flagellates Cartesia obtusa and Astasia, rotifers Alosa worallii and other inhabitants of sphagnum mosses living in water with pH up to 3.8-6 and not found in neutral and alkaline waters).