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Do fish pee?

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When I was doing research on the aquarium’s nitrogen cycle, there were a lot of articles talking about fish feces being a big source for ammonia diluted in your water. This raised a question that I had never thought of: do fish pee? It differs on the species of fish, but even more on the fish’s habitat. Here is why.


Fish handle their waste in different ways depending on the environment that live are in. Are they in salt or in fresh water? This is import because of osmosis has a massive impact on the fish. Depending on osmosis, a fish pees constantly or a fish pees hardly at all. What is osmosis?

Osmosis is the process when water moves through a membrane from a low concentration of solutes to a high concentration in order to make the water concentration equal on both sides. I included the definition of “osmosis” below according to . Osmosis happens constantly at the skin of a fish, it can be seen as the semipermeable membrane in the image above.


The tendency of a fluid, usually water, to pass through a semipermeable membrane into a solution where the solvent concentration is higher, thus equalizing the concentrations on either side of the membrane.

In the case of aquariums the solvent concentration is salt. Freshwater fish are much saltier compared to the water they are swimming in, and for salt water fish it is the opposite.

Fresh Water Fish

So what does osmosis mean for fresh water fish? The water that they swim has a low concentration of salts, and the inside of the fish has a high concentration of salts.

If we take a look again at what osmosis does, we see that the water wants to equalize the water concentration of both inside and outside the fish. This means that water is constantly flowing from the surrounding water to the inside of the fish. There is nothing a freshwater fish can do to stop this, so it has to get rid of the water again.

In order to get rid of all this water they almost constantly urinate. If they would not do that, their cells would keep swelling up like a balloon filling with water and possibly even burst. The urine of a freshwater fish is heavily diluted but does contain substances like ammonia and phosphate. You can compare it to human’s pee when they drink a lot of water. It is clear and all the waste products are heavily diluted.

Chloride cells in freshwater fish

A fish needs a certain degree of salts, just like we humans do. If the fish almost constantly pees this also mean that they will loose a lot of crucial salt ions. To secure the required amount of ions, the fish makes use of special cells in its gills called chloride cells that pump sodium and chloride ions directly into the fish’s blood. These chloride cells help freshwater fish prevent a shortage of crucial ions because of all the water they have to flush out of their system.

Salt Water Fish

A salt water fish lives in water that is very salty (obviously), much saltier than the inside of the fish! This means that it is harder for a saltwater fish to maintain enough water inside its body because the water is constantly flowing from the inside of the fish to the salt water outside the fish due to osmosis. They loose a lot of water this way. That can not be good if you are a fish that wants to survive!
To compensate for the loss of water, a saltwater fish is actively drinking water, which is something a freshwater fish would never do. A saltwater fish has to do so to obtain enough water in order to keep the water level inside its body sufficient. But what does the fish do with all the salt that it consumes with the water that it is drinking? The kidneys can only handle a small amount of salts, surely it can not be good for the fish to take in all the salts that come with the water it is drinking. Luckily saltwater fish also use chloride cells.

Chloride cells in saltwater fish

Well, the chloride cells that were making sure to pump ions into the blood that were located in the gills of a freshwater fish are now working in full reverse. They remove a lot of salt from the water the fish consumes and “pump” it out back into the ocean/aquarium water through the gills. Because of these chloride cells a saltwater fish is able to drink a lot of water and maintaining the relatively low salt concentration inside its body at the same time.

This does mean that a saltwater fish pees way less that a freshwater fish, because it has to be careful with the water that it has and should not waste it. The urine of a saltwater fish is heavily concentrated with waste products such as the ammonia and phosphates. This again is comparable to the pee of a human being when it is dehydrated, instead of the clear fluid the pee now is dark.

Special cases – Salmon and Sharks

There are cases that, given this knowledge, are exceptions. Think of a salmon for example. These animals are incredible osmoregulators.

Salmons are born in rivers where they spend the early life. They swim to the ocean where they in turn gain most of their mass and live their lives. When they are ready they migrate back to the river during the fall, and they spawn. Pacific salmon have no energy left after spawning because they stop eating when they swim up river and therefore they die. The Atlantic salmon do however not die after spawning and have energy left to make it back to the ocean. Therefore they can repeat the spawning process multiple times in their lifetime.

Salmon excel at controlling their body fluids and ions during the incredible transition from fresh to salt water and back again. They have to undergo severe biological changes in their body to be able to do this.

This means that the new-born salmon are living in fresh water, and because of osmosis we now know that their kidneys will produce a lot of urine, they do not drink extra water because they experience no shortage and the pumps in their gills (the chloride cells) pump ions into their body.

When the salmon are ready to swim back to the ocean, their body will under a change in order to prepare the salmon for life in salt water. The salmon will have to start actively drinking water, and the kidneys have to severely reduce urine production. The chloride cell have to start pumping out ions back into the water. These are all the 3 traits from fish that live in salt water.

When the fish is fully accustomed to the salt water it eventually will migrate back to the rivers it once was born. Their body will undergo the same transformation in reverse.

Osmoregulators and Osmoconformers


All the fish that we have just discussed are all osmoregulators. Now I know that term sounds difficult, but do not worry. After I did some research I am confident you will understand it soon, let me explain.

All osmoregulators have a different ion concentration in their body compared to the water they are swimming in. A neon tetra is saltier than the aquarium water it is swimming in. To make up for this it uses the chloride pumps in its gills.

The concentration is called the “osmolarity”. Which according to means:


The osmotic conentration of a sulution expressed in osmoles or solute per liter of solution.

Some examples of fish that are osmoregulators are the salmon we discussed, but also almost all freshwater fish together with a lot of saltwater fish.


Sharks are examples of osmoconformers

Osmoconformers are animals like lobsters, jellyfish and sharks. Their body has the same osmolarity as the water they are swimming in. They conform their body’s ion concentration to match the osmolarity of the water that they live in.

Lobsters, jellyfish, hagfish and many more marine invertebrates use the same ions that are readily available in the seawater they live in to conform the osmolarity of their body to match their surroundings. The animals that do this are called ionconformers.

Sharks are osmoconformers because the osmolarity of their body matches the osmolarity of the water, but they, unlike the ionconformers, do not use the same ions that are present in the water they swim in.
Sharks are not ionconformers but ionregulators, they use different ions to match the sea water’s concentrations. They also have to deal with a slight abundance of salt, which they excrete by a rectal gland.

The ion that sharks use to raise their body’s osmolarity is called urea. This ion is present in many organisms, but most of the animals eliminate it from their body through urine. Sharks do not, they use it to their advantage.

Urea is also the reason that sharks smell like ammonia when they die.


Well, fish pee! I thought it was absolutely fascinating to learn why freshwater fish pee a lot and saltwater fish hardly pee at all. It is amazing to learn that salmon undergo severe biological changes to spawn high up in freshwater rivers.

I hope you also found it interesting to read, and that you learned something new. If you want you can subscribe to the mailing list on the right in the sidebar. If you do, I will make sure to notify you when I release a new article that hopefully sparks your enthusiasm for our hobby.

There are some articles below that you can read if you want, I personally would like to recommend our article on the nitrogen cycle next. You can press green button below and it will take you straight to it!

Thanks for reading!


Banner image – Author: LEONARDO DASILVA Licensed under CC2.0 no changes
Osmosis image – Author: Openstax Licensed under CC4.0 no changes
Rummynose tetra image – Author Gavinevans licensed under CC3.0 no changes
Salmon image 1 – Licensed under CC1.0 no changes
Salmon image 2 – Licensed under CC1.0 no changes
Starfish image – Author mrpenguine licensed under the Pixabay License no changes
Shark image – Author Steve Garner  licensed under CC2.0 no changes

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