Osmotic pressure

What is it?
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
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.

Osmoregulation in marine animals
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.

Freshwater animal treatment
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.

Marine animal treatment
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.

Links

 * Wikipedia on Osmotic pressure
 * Ionic Balance and Osmoregulation in Fish
 * Osmoregulation: Please give me a drink!
 * Osmoregulation and fish health by Fishdoc