What Happens to Freshwater Fish in a Salt Water Tank?

According to Dr. William Wurts Ph.D., professor at Kentucky State University, all fish species originated from the planet's oceans. Over time, species of fish began to tolerate lower concentrations of salt in the water. The evolutionary changes eventually led to the transition of fish to fresh waters. If you were to introduce a freshwater fish into a saltwater aquarium, the fish would die a painful death. However, one species of freshwater fish has the ability to adapt to a saltwater aquarium.

  1. The Body's Reaction

    • Through the process of osmosis and diffusion, freshwater fish naturally absorb more salt than water through their gills. Freshwater fish hold salt within their cells and urinate excess water to remove salt from their kidneys. In this way, freshwater fish sustain the proper balance of salt and water in their systems. According to fishwaterchannel.com, when saltwater enters through gills of a freshwater fish, the cells become flooded with water. As the fish swim, saltwater forces the cells to lose water. This occurrence is known as osmosis, where a high concentration to a lower concentration can lead to the dehydration and eventual death of a freshwater fish.

    Struggle to breathe

    • With salt entering its gills and flooding the cells, a fish will begin to struggle to breathe. As this occurs, oxygen decreases and the fish begins to suffocate. A freshwater fish will move to the surface of the aquarium and gulp air in an attempt to gain oxygen.

    Erratic Swimming

    • A freshwater fish will swim back and forth, attempting to increase the amount of water entering its body. By quickly darting and moving fast it tries to rush oxygen over the gills to breathe. According to fosterandsmithaquatics.com, a fish swims erratically to pass more water through its gills to absorb saltwater. Unsuccessful, the fish will begin to move back and forth and soon lose its ability to stay afloat.

    Death

    • A freshwater fish's body will shut down, as vital organs cannot function as saltwater enter its cells. The kidneys of a freshwater fish are not able to remove enough of the salt entering its body and they begin to fail. With the abrupt change in the molecular composition, cells begin to expand and the fish becomes bloated. The fish is unable to remain upright and floats to the top of the aquarium, where it lies helpless as its respiratory systems struggles to create air. Eventually, it will die.

    Species adapting

    • According to freshwater-aquarium-fish.com, a Mollie can become accustomed to living in saltwater. Mollies' bodies can react to changes in salt by adjusting their intake of salt by excreting more water through their kidneys. An aquarium owner can adapt a Mollie to a saltwater environment by placing the fish in a small container and gradually apply saltwater to the freshwater. Eventually the Mollies' body will adjust to the saltwater. A Mollie is a rare example of a freshwater fish surviving in saltwater, if given the opportunity to acclimate before placement in a saltwater aquarium.