A Yellow Trail: Do Fish Pee? How, Why & When

do fish pee

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I was enjoying a hot cup of coffee while watching my favorite nature show on TV when the narrator casually mentioned that fish pee.

I was skeptical, to say the least. I’m sure that I wasn’t the only one who felt a little incredulous and was waiting for some kind of punchline or prank announcement, but it just didn’t come.

“Fish pee?” I thought to myself as my brows knit together in confusion. “What on Earth is my favorite nature show trying to tell me? How do fish pee when they don’t even have a genus to urinate from?”

Let’s find out…

Do Fish Pee?

I was driven by curiosity, which led me to conduct some research on the topic and I found out that there are all sorts of fish that pee.

There are specific types of fish that produce urine. Fish will pee for their bodies to be rid of waste, and some even do it for a certain purpose.

For example, male tangs send out a chemical that sends females away to go into a mating frenzy.

What is fish pee? Fish pee is just urine but it isn’t exactly like our own urine. The main difference is that algae and many other chemicals in the water cause fish to have a “pee-pee” or “urine.”

As you know, fish and the water they swim in are filled with living organisms. 

These organisms create a food chain, which is important in maintaining the health and balance of each habitat.

However, fish are constantly complaining about how messy these organisms are. They often blame them for their dirty bodies as well as their smelly smells.

So, they decide to use urine to lighten up the chemical load that these organisms leave behind on their bodies.

As the fish swim closer to the surface, they start to pee. The urine is released into the water and is collected by other organisms to survive.

There are fish that do not produce urea and uric acid, which is what we typically think of as pee. These types of fish release only ammonia and ammonium salts.

This helps the fish digest their food properly and also helps keep their body balanced and healthy.

How Often Do Fish Pee?

Fish pee all of the time, but not just anywhere. Fish will only release their urine when they are swimming at or near the surface because it is less dense than the water below.

In fact, according to an article on Wikipedia, some fish pee when they can no longer stay still to maintain their position in a school of fish.

Some fish pee nearly every time that they are approached by another fish.

Many people believe that fish have to be distressed to pee, but this is not true at all.

Fish do not feel the same way that we do, so they pee whenever they need to to stay healthy.

When a fish feels threatened or is unhappy, it will likely pee for no reason at all.

How Do Fish Pee?

There are a variety of ways that fish pee, but there are basically two types. Besides the “pee-pee,” or urine, there is also the “lipid-rich filtrate” or LRF.

The LRF is simply a metabolism byproduct of the fish’s body. The term LRF is very misleading as it implies that pee is not in there at all.

This misconception is what has led to some rather odd depictions of how a fish does its business.

The “pee-pee” or urine is the secretion of a fish called the epithelial cells.

Fish pee through their gills as they swim near the surface through a tiny duct located in the roof of their mouth. The epithelial cells are designed for this process and the assembly of mucus.

The urine itself is stored in what are called nephridia which are not like bladders at all. It is usually not visible to the naked eye and it is usually just kept in a sac between their scales.

The secretion does not emit any odor, which may be why people have mistaken it for water in the past.

What Does Fish Pee Look Like?

It is nearly impossible to know what fish pee looks like as it is released into the water.

However, it can become visible as it is collected by other organisms to survive.

You may see a thin outline in the water, which is a thin line of white that creates an obvious visual breakdown of the organisms that purify the water.

The urine itself is usually clear, but it may have a little bit of green to it. 

In a way, fish pee is like sweat for humans. When we are hot or under stress and we sweat, our bodies are signaling that something isn’t right with how we are acting or reacting to our environment.

The same thing goes for the fish, the chemical load that their bodies are carrying is just too great, so they release urine to help their bodies stay healthy.

Fish pee can be seen when it collects on the fins of some types of fish as well. For example, if you see a small spot of blue on a fish’s tail fin, this may be an indicator that it has just peed.

This is not true in all cases as many fish will pee directly into the water around them and this will also collect in their fins.

What Are The Benefits Of Fish Pee?

Fish pee is just one of the many ways that fish and other aquatic organisms stay healthy and happy.

This waste product allows these types of creatures to live in habitats that are not friendly to us, yet they are all able to survive.

While it may seem disgusting to think about, there are many benefits to fish pee.

Fish pee helps balance the water’s PH level, which is important for a variety of reasons. It can also provide nutrients for other species of aquatic life, as well as plants and trees.

The urine itself is nutritious for some types of organisms that live in the water around us.

Facts About Fish Pee

  • Did you know that there is a type of fish that doesn’t pee? These fish are called spiny eels or “bonytongues.” Instead of peeing, they make a mucus layer on their body to keep them moist.
  • You may be able to see some blue stains on the tails of some fish. This could be a sign that they are peeing, but not all fish that have urine in their fins also have urine in their mouths.
  • There is an ocean that has more than a million gallons of fish pee in it. This is the Great Barrier Reef and it is located in Australia. The reefs here provide a lot of nutrients for other marine life and humans alike, which allow all different types of organisms to thrive and live in the area.
  • The oceans and the creatures within them cover more than 70% of the earth. This means that there could be a lot more fish pee than we think.


Do fish pee? Yes, they certainly do! Now that you know more about fish pee, you will likely look at fish in a whole new light.

It may seem gross to think about the bodily functions that any type of animal has.

However, for them to survive and thrive, they must have the waste products of their bodies removed from their system before they accumulate too much. Fish pee is one way that they accomplish this task.