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Quenching the Curiosity: Do Fish Get Thirsty?

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While we know fish, and all other living creatures, need food in order to survive, a common question is whether or not fish need to drink the water in their tank because they are thirsty. 

Freshwater fish do not need to drink water, nor do they feel thirsty. Freshwater fish absorb water through their skin. Therefore, they actually have the need to eliminate urine constantly. While fish sometimes gasp the water, they are not drinking it but rather trying to get oxygen in their system. 

While freshwater fish do not drink water, this does not hold true for their saltwater counterparts. There are a few scientific reasons why this happens so keep reading to learn more!

Do freshwater fish get thirsty?

The fact that freshwater fish do not need to drink water is due to the process of osmosis. Fish absorb the water through their skin and gills. In other words, the process of osmosis is overall the flow of water across the membranes with the lower concentration of solutes (in this case salt) to the areas with the highest concentration of these same solutes.

To further explain, the bodily makeup of a freshwater fish is saltier than the water they are in. Therefore the water will flow through their gills making them need to urinate more often. They are constantly hydrated.

With that being said, no. Freshwater fish do not get thirsty. However, saltwater fish do!

Green beautiful planted tropical freshwater aquarium with different fish

Saltwater Fish and Osmosis

Osmosis affects saltwater and freshwater fish differently. While freshwater fish do not need to drink water purposefully, saltwater fish do! Most fish that live in the ocean, or in saltwater aquariums lose water at an alarming rate.

This is because salt water has a very high concentration of salt and this water constantly flows out of the saltwater fish’s gills. They in turn can dehydrate quickly and shrivel up. In order to stay hydrated, they need to drink a lot of water throughout the day.

So if a saltwater fish needs to drink water because their bodily autonomy is also salty and they are dehydrated: doesn’t drinking salt water defeat the purpose? The answer is no! 

Saltwater fish actually have kidneys that pump the excess salt into their urine. This is why saltwater fish do not urinate a lot like freshwater fish, but their urine is highly concentrated. 

This means that not only do they pump out excess salt through their kidneys, but they also pump out extra salt through their gills.

The gills of a saltwater fish have specialized cells, as opposed to a freshwater fish, that allow the fish to automatically get rid of the salt. In other words, the gills of saltwater fish are constantly pumping salt out, while the gills of freshwater fish are constantly pumping salt in!

Exploding Fish Phenomenon 

Let’s say both freshwater and saltwater fish did not have specialized cells in their gills; what would happen? This is simple.

If the cells kept pumping salt into the saltwater fish, instead of pumping the salt out, they would quite literally just shrivel up due to dehydration. On the other hand, if a freshwater fish kept pumping all of the salt out of the fish it would explode due to too much water intake. 

Osmosis Experiment

While osmosis can be hard to grasp without a visual representation; there is one way to see the effects and that is with a potato! This ia great experiment to do with kids as well. You will want to cut up a few slices of potato to start.

You should then grab two small cups to leave on the counter overnight. One should contain freshwater and the other should have saltwater. Places a slice (or two) in each of the cups and leave them alone. 

When you check in the morning, you will notice that the potato in saltwater will be dehydrated, while the other in freshwater will have absorbed most of the water and will be much softer. 

This is exactly what happens to the fish, however their body has a way of maintaining the balance of water and salt in their bodies to make sure they don’t dehyrate or explode!

Keeping a Freshwater Tank

Freshwater tanks house some of the most beautiful pets! As we now know, freshwater fish do not drink water but they do release a large amount of urine. This is why it is extremely important to always test the parameters of your water.

Urine holds bacteria and too much of it can cause a build-up of ammonia and end up killing your fish. It is recommended that you test monthly, or weekly if you have a larger amount of fish in your tank. 

Keeping a Saltwater Tank

Saltwater fish do need to drink a large amount of water daily in order to stay hydrated since they are absorbing saltwater. With that being said, the tank water is what they drink so it needs to be kept in pristine conditions to ensure that the fish do not get sick. 

Make sure you do weekly cleanings and have a great filter in place to cut down on waste in the tank. This ensures the fish are drinking cleaner water and will overall be healthier. You should also do weekly testing strips to test the parameters of the tank.

On top of this, you can add a few bottom feeders to your tank to help with the cleanliness of the water. Adding shrimp or snails are also a great option to combat extra waste in the tank. 

Do fish pee?

As mentioned, freshwater fish naturally absorb the water around them and therefore need to urinate frequently. So yes, fish do pee! An interesting fact to know is that unlike humans and other animals that pee multiple times per day, freshwater water fish will have a constant slow stream of pee throughout the day! 

On the other hand, saltwater fish will only urinate about once per day. However, as opposed to freshwater fish, the urine of a saltwater fish is heavily concentrated due to all of the salt running through their bodies.