Water is the natural environment for the life quality of fish and hence water quality is of prime importance for fish production and the quality of fish as well. Even in aquatic eco-systems, one can simply say “what goes around comes around”. Understanding how to improve the conditions in fish farming in terms of production and quality have very much to do with understanding the functioning and metabolism in natural aquatic systems. There are key issues that are essential to be understood about water quality in aquatic systems (fresh, brackish and marine), in particular the physical (e.g. density, stratification and mixing as well as turbidity, mineral particulate, light transmission) and chemical (e.g. pH, solubility of gases such as oxygen, carbon di-oxide, ammonia, salt concentration, dissolved matter, organic and inorganic particulates) properties and characteristics in these waters at different temperatures. The physical and chemical conditions of water determine to large extent the water quality status, along with toxic anthropogenic compounds that originates from pollution and water from industrial, agricultural and household sources. The impacts of all these conditions, factors and properties on water quality, the response and feedback effects on fish in terms of production and quality are to large extent summarized in the following document, it can be used as a guide for fish farmers.