Fish, Tanks and Ponds

Fish, Tanks and Ponds
A comprehensive guide to fish

Osmoregulatory system

Giant goby, Gobius cobitis
Giant goby, Gobius cobitis


In osmosis a weak solution will always try to invade a strong solution and dilute it until a balance is reached. The greater the difference between the strength of the two solutions the greater the osmotic pressure.

This has implications for both fresh water fishes and salt water fishes. In the case of fresh water fishes their environment is usually a weaker solution than their internal bodily fluids and so water is constantly trying to invade their body. The opposite is true for salt water fishes where the surrounding sea water is a stronger solution than their bodily fluids and has a the effect of drawing out the fishes bodily fluids causing them to dehydrate.

Fresh water fishes

Fishes living in fresh water have to be able to get rid of large amounts of fluid from their body because in most cases  their surrounding environment is made from a weaker solution than their internal bodily fluids which means that water is constantly invading their body. To combat this fresh water fishes don't need to drink and they have kidneys which process large amounts of fluid and they are able to pass a relatively high amount of very dilute urine.

Because their urine is so dilute it means that they are able to retain the important salts while getting rid of a lot of excess fluid.

Salt water fishes

Fishes living in a salt water environment face the opposite problem to fishes living in fresh water because the osmotic pressure works in the opposite direction because their bodily fluids are less concentrated than their surrounding environment  and thus marine fishes inhabit a hyperosmotic environment and experience continual dehydration. Water loss is compensated by imbibing the medium and absorption of the sea water in the gut. Excess salt (Na + and Cl −) is excreted via the ‘chloride’ cells or mitochondrion rich cells in the gills whilst calcium is excreted as a calcium carbonate precipitate via the gut.

Elasmobranch osmoregulation

Elasmobranch fishes (Sharks and rays) are able to concentrate stored urea in their blood at a slightly higher concentration than their surrounding environment which greatly reduces the osmotic pressure on their system and eliminates the need to constantly drink as marine teleost have to do. But salt still enters their body through osmosis and this is regulated by a gland close to their cloaca called a rectal gland. A few sharks are able to enter estuaries and even fresh water where their bodies adapt and behave like a fresh water teleost. Bull Sharks Carcharhinus leucas in particular are very adaptable in this respect and are able to live their entire life in fresh water

The Osmoregulatory system

There is no single osmoregulatory organ in fishes but several different organs have a role to play.

  • The gills
  • The kidneys
  • The endocrine system
  • The rectal gland (Elasmobranchs)

The gills

The gills of a fish is were gasses are exchanged but also where most osmosis takes place too. The fish must take measures to counter this in order to maintain the right ionic balance in its bodily fluid.

The lamellae is the part of the gill filament where gas exchange happens, the lamellae are held apart by pillar cells which are controlled by the endocrine system and can be made either thicker or thinner in order to regulate gas exchange and ionic transfer.

Despite this control it is the gills which are largely responsible for allowing most of the fluid in to the fishes body because for the gills to work  properly for respiration it is not possible to prevent the up take of other fluids too and for them to still work efficiently.

The kidneys

Most fresh water fish have two relatively large kidneys which have evolved to process relatively large amounts of dilute urine and to hold on to the important bodily salts. Most mammals produce 1.5% of their weight in urine each day, a fresh water fish produces 20% of its weight each day. While it is the gills where most of the excess fluid enters the fish it is the kidneys which get rid of most of the excess fluid.

The endocrine system

Various hormones control how much a fish will drink, how much urine it produces and how permeable the gill lamellae are by increasing or decreasing the gaps between the cells in the wall of the lamellae.

The rectal gland

Is a gland found in elasmobranchs (sharks and rays) which excretes excess salt which builds up under to osmotic pressure.

What this means for fish keepers

Most fish have a range of conditions which they can adapt to, some fish are very adaptable while other have to live within a small range of conditions and will die prematurely if kept outside that narrow range. It is a myth that all the fish bred commercially can adapt to 'ordinary water' in fact most can but some can't for the reasons pointed out in this article. Black mollies must have hard alkaline water or a small amount of marine salt added to their water if they are to thrive, Rift valley fish must be kept in alkaline water and are not suitable for general community aquariums. It is important that the prospective fish keeper does some research (other than asking the vendor) before buying any species of fish.

Disease treatment with osmosis

Marine fish keepers use this principle to their advantage by using a fresh water dip to kill external parasites on marine fish.

The parasites are less able to cope with a change in osmotic pressure than are marine fish.

So when they are placed in fresh water the parasites take on more fluid to such an extent that it kills them, although stressful to the fish, such a dip is rarely harmful in the long term.

Osmosis and equipment

Most of us will have heard about reverse osmosis units, but have you ever wondered how they work?

Water is forced under pressure through a semi permeable membrane but only water is allowed through and nothing else which means that water is going from a strong solution containing all the dissolved solids to a weak solution of just water, this is the opposite of what normally happens and that is why it is called reverse osmosis.

r/o water is to pure to keep fish because the solution is so devoid of dissolved matter that the osmotic pressure on the fish is normally to great and they end up like the fish in the example where water ends up invading its body and the fish is unable to keep up with getting rid of it. 


Osmosis - is the net movement of solvent molecules through a partially permeable membrane into a region of higher solute concentration.

Osmotic pressure - Osmotic pressure is the pressure which needs to be applied to a solution to prevent the inward flow of water across a semipermeable membrane.

Osmoregulatory - is the active regulation of the osmotic pressure of an organism's fluids .

Elasmobranch - A group of fishes with cartilage skeletons which include sharks, rays and rat tails.

Teleost - modern ray finned, bony fish.



Wikipedia - Osmosis.
Wikipedia - Osmotic pressure.
Europe PubMed - Osmoregulation in fish.
People Biology - Ionic Transport in the Fish Gill - A Review of Osmoregulation in Fresh Water and Marine Elasmobranchs.
By Neil Hammerschlag

Aquarology Master Volume
Edited by Dr John B Gratzec, and
Ms Janice R Mathews
Pages 179 - 183

Eddy, F. Brian; Handy, Richard D. (2012-05-03). Ecological and Environmental Physiology of Fishes (p. 79). Oxford University Press. Kindle Edition.