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Understanding Osmoregulation


So you know that saltwater fishes cannot survive in freshwater tanks and you have to make it alkaline before making the tank ready for your fishes. But do you know why? Some people complain, especially the newbie that some of their their fishes die despite the aquarium being apt for the other breed of invertebrates. The answer to both the questions lies in understating the process of osmoregulation in fishes.

Ever wondered why it is advised to pay heed to the origin of the fishes? It is because different fishes coming from different origins are compatible to different level of alkalinity will not be able to adapt to your tanks water composition which might differ vastly leading to weakening or death of these fishes. Always take care of this factor if you want them to be healthy. Osmoregulation system maintains a stability of salts and fluids inside the saltwater fishes. The greater amount of salt within the water makes the osmoregulation system work harder to correct the salt levels and minerals inside the fish creating higher osmotic pressure. A fish can tolerate different saltwater levels depending on the environment the fish has evolved from.


A freshwater fish normally sustains its inside salt level higher than that of the water in which it is living. The gills take in salt from the environment along with oxygen. A large amount of water is continually diffused into a fresh water fish and excreted as urine so that its system is always being flushed. In contrast, a marine fish has an inside salt level lower than the water in which it is living so the osmotic pressure naturally pulls water out of the fish and pushes salt in. Therefore, to maintain the accurate salt level, a salt water fish needs to actively excrete salt out of its gills. This explains the process of osmoregulation in fishes.

Fishes of freshwater can withstand only a very low osmotic pressure compared to saltwater fish which are used to surviving only under a higher level. Some fishes that travel from freshwater rivers to saltwater seas are quite vulnerable and they can adapt to the change in osmotic pressure. They are called brackish water fish. Brackish water fish’s body is quick to respond to the change in osmoregulation. The osmosis process of balancing salt and water in the body of these fishes can alter variably. If they have been in freshwater and swim across to some saltwater source, the processes of osmoregulation quickly responds to the intake and excrete of salt and water from the body so as to maintain the equilibrium of osmosis.

If you find your fishes stressed in the initial days after adding to a new tank, you must start to lower the salt content in water. But remember to increase the salt content in a very small quantity. A sudden increase or decrease can make it difficult for the fish’s osmoregulation system to correlate resulting in death. When you see it is recovering, very slowly increase the salt content in water. In a day the salt content should never be increased or decreased more than 0.002%.

At the same time keep an eye on the corals in your tank. During the lowering of salt level, the corals may refuse to open up during day. But that is not going to cause any harm generally. If you find the corals responding aversely to the salt decrease, it only means that you made more than necessary change suddenly. A slow and gradual lowering will not cause any harm and the corals will be seen healthy with the increase of salt later. You can also use a quarantine tank for new fishes to adjust slowly without altering the environment of other inhabitants of the tank.