Osmotic pressure

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What is it?[edit | edit source]

Osmotic pressure is the natural force of minerals dissolved in water moving across a semi-permeable membrane into another body of water containing less minerals in an effort to equalise.

This difference between the two sides is the 'pressure'.

This natural force is used to good effect in producing clean and mineral-less water via a process called RO (Reverse Osmosis).

The membrane can be any thin 'skin' that allows water to pass through it but not ions. So this includes animal cell membranes. So osmotic pressure happens when you transfer a live animal from one body of water to another.

If the difference between the two bodies of water is very great and the animal is not built to cope with a sharp change then the animal can die due to Cytolysis (cell wall bursting). This is called Osmotic shock.

Osmoregulation in freshwater animals[edit | edit source]

A freshwater animal in a river has its cells holding a saline solution close (but lower) to the density of sea water and as the river water is of far less osmotic pressure than the fish it is therefore constantly losing minerals to the river water.

To replace these, the fish has to constantly swallow river water with its minerals and let its kidneys remove the minerals and pass them onto its cells to replace what has been lost. It then has to urinate constantly to get rid of the excess of water.[1]

Osmoregulation in marine animals[edit | edit source]

A marine animal on the other hand does the opposite. It's cells are also slightly lower in concentration than sea water and so the sea water is trying to force minerals into the cells.

Therefore it is constantly having to drink sea water, filter out the minerals and so dilute the build up of minerals in its cells. It then has to urinate salty water (called urea) constantly to get rid of the excess water to stop becoming hydrated.


This regulation doesn't happen overnight, it can take 48–72 hours for an animal to readjust. So taking a shop bought fish home in a bag with one water type to another in your tank can not be accomplished in 20 minutes (the usual time often quoted to acclimatise fish to your aquarium water).

Often the effects of a sharp change of osmotic pressure is only seen after a couple of days when the animal dies or seems quiet.

Adjusting Osmoregulation for treatment[edit | edit source]

Freshwater animal treatment[edit | edit source]

Aquarists can temporary adjust the osmotic pressure in the aquarium to aid a fish when it is unhealthy. In freshwater fish or amphibians we all salt or epsom salt to the water so increasing the mineral content of the water so it more closely matches the animal. This makes less work for the animal internal organs and so can aid it back to health. But you must not of course exceed the saline content to more than the animals internal level or it will be unable to remove the minerals and you will harm or kill it.

  • High levels of salt are occasional used to kill off external parasites. But the animal is only dipped into this water for brief periods.

New owners of the popular freshwater invertebrate Triops often are instructed to hatch out the creatures in near pure water and then they may decide to transfer the animals into an aquarium. Often the result is death by the second or third day. But a simple slow exchange of aquarium water from the pure water over 3–4 days is all that is required.[2]

Marine animal treatment[edit | edit source]

Aquarists can temporary adjust the osmotic pressure in the aquarium to aid a fish when it is unhealthy. In marine fish we dilute the salt content to the water so decreasing the mineral content of the water so it more closely matches the animal. This makes less work for the animal internal organs and so can aid it back to health. But you must not of course decrease the saline content to less than the animals internal level or it will be unable to recover the lost minerals and you will harm or kill it.

  • Low or zero levels of salt are occasional used to kill off external parasites. But the animal is only dipped into this water for brief periods.

Notes[edit | edit source]

Often oversea trade suppliers will ship their stock of freshwater fish in slightly salty water to reduced the stress of the travelling process on the animal. The local import distributor will have quarantine tanks that contain mildly brackish water matching the suppliers water and so the newly imported stock will need a few days of being adjusted back to local freshwater water before they can be put on sale or being passed on and resold to the local aquarium shop.

  • It also allows (or should) the local distributor to monitor any diseased fish and treat them before selling them on.
  • Each species of fish contains its own level of internal saline content. So treating a mixed species tank with salt is risky.
  • Some freshwater or marine fish may live near the mouth of a river or have to live in either environment and so have learned to develop an altering level. See Trout or Eels for an example. The popular freshwater invertebrate, the Amano Shrimp lives in freshwater. But its young need to live in brackish water briefly to mature.
  • Since the 1950s it's been technically possible to make aquarium water which has an osmotic pressure similar to the internal pressure of freshwater or marine fish as they are very similar. By reducing the saline and adding chemicals to alter the osmotic pressure you can make the two groups of fish live together in the same tank.[3][4]

Links[edit | edit source]

References[edit | edit source]

  1. Osmoregulation in freshwater fish by the Food and Agriculture Organization of the UN
  2. Transferring Triops into a big tank - MyTriops.com
  3. Patent detail #1 and Patent #2 on how to achieve co-existence between freshwater fish and saltwater fish
  4. Wonder Water - Fresh And Saltwater Fish Living Together (archived link) by Kordon