Freshwater fishes are those fishes who spend most or all of their lives in fresh water with a salinity of less than 0.05% such as lakes and rivers.
This Fresh water environment is different from marine conditions in various ways which the most obvious difference is the level of Salinity.
For fish to survive in fresh water, it needs a range of physiological adaptations.
About 41.24% of all known species of Fishes are found surviving in fresh water and this is primarily due to the rapid speciation that the scattered habitats make possible. One might use the same basic models of speciation when dealing with ponds and lakes as when studying island biogeography.