In terms of water salinity, freshwater is 0. 05% salty, brackish water is 0. 05-3%, saline water is 3-5%, and brine is >5%. The gram weight of solute is relatively constant in sea surface salinity. However, the arctic/antarctic ocean is said to have slightly less than average salt content, much like the salt content of the. Ion ratios in salt water are also relatively stable. Hard water is defined as water of low salinity, often derived from the drainage of calcareous earth. Soft water has lower salinity and is derived from acidic igneous rock. In addition to exchanging gaseous co2 and o2, gill surfaces are used to take up salts from the environment and process a large volume at that, even though there is not much nacl in freshwater. The freshwater fish"s very dilute urine gets rid of excess water without losing too much salt.