Freshwater fish absorb water passively and release it actively. Meaning they acquire water using no energy, but in order to release it they need to use energy. This process is due to fish's kidneys. Their kidneys gain water osmotically. Meaning they absorb water by osmosis, which as I touched on before, is a type of passive transport. Fish's kidneys also produce dilute urine in order to get rid of waste matter. Fish are in a constant state of osmosis, because they are hypertonic. This is not good because water continues to diffuse into fish, creating a very high concentration of water in their system. Fish deal with this problem with their kidneys. Fish's kidneys have a big surface area which enables them to reduce the concentration of water in their blood. However in order to do so they need to release important ions out of their bloodstreams, making it necessary to absorb ions such as salt back in. These two functions require energy meaning they use active transport.
Sources:
1) http://www.biology-online.org/5/1_animal_water_regulation.htm
2) http://www.cartage.org.lb/en/themes/sciences/zoology/animalphysiology/osmoregulation/osmoregulation.htm
3) http://en.wikipedia.org/wiki/Fish
4) Image http://www.incikefali.net/uremegocu-en.htm
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