how do fish poop

Although many people keep fish as pets, they often don’t think about fish biology. Since there is no visible orifice aside from their mouth, you might not notice some of their habits, such as peeing and pooping. However, fish are just like any other living thing and have biological processes and demands, such as consumption, digestion, and excretion.

But how do fish poop and pee? Is it the same way as other animals? And how does fish pee and poo affect an aquarium?

Keep reading to find out.

Do Fish Poop and Pee?

pooping fish

Let’s get the most pressing issue out the way: Do fish poop and pee? As mentioned earlier, fish are living creatures that need to get rid of their waste. No matter where fish dwell, they are urinating and excreting feces into the water around them.

Sometimes, this can help maintain a balance in an ecosystem, such as a barrier reef. Other times, such as with a tank, poop and pee from your fish can become detrimental and will need to filtered out consistently to prevent algae growth and other issues.

Chances are that you have seen fish waste and haven’t noticed it. We’ll get into more detail about that later!

How Do Fish Poop and Pee?

peeing fish

Now for the how. While fish do poop and pee, the process is a bit different from mammals.

Fish Pee

Like humans, fish have a pair of kidneys. Different types of fish have different sized kidneys, but all kidneys do the same job of removing urine and waste from the body.

Fish don’t have to pee as often as people do, but they do pee throughout the day. This is because fish kidneys process waste more slowly. When it comes time to pee, the liquid will sometimes exit from a small orifice by the tail; sometimes, it is realized through the skin and gills, too.

Another difference is the bladder. Humans have a bladder that holds urine, but fish don’t. Instead, urine moves from the fish’s kidneys and out the body. Their swim bladder is used for buoyancy.

If you are curious to whether your fish are doing their business or not, you can just take a whiff of the water in the tank. That ammonia you smell? It’s from urea—or pee. This is why aquarium water smells horrible if you wait too long to change it.

Freshwater Fish

Freshwater fish will excrete water while retaining salt, which is needed for optimal functioning. Without salt, freshwater fish cannot grow properly. Because of this, freshwater fish had kidneys that will cling to any salt while letting water pass by. This creates watery urine without much of a scent that passes through the posterior, by the anus.

Saltwater Fish

Marine fish have a different system, because their environment is different. This means that they are the opposite to freshwater fish and will excrete salt while retaining water. Therefore, their urine consists mainly of salt, making it very concentrated. Also, marine fish do not pee as often as their freshwater counterparts. In fact, saltwater fish may pee once a day or every two days. Saltwater fish can also excrete salt through their permeable bodies and gills.

Fish Poop

Yes, all fish poop. When a fish eats, the food is processed by the digestive system, and it eventually moves through the intestines and out the anus. The process is pretty slow. Fish will usually excrete poop once every 48 hours.

If you see a lot of pellets around the substrate, that is probably poop and not uneaten food. Other fish have sticky or stringy poop that will cling to plants and the side walls of the tank. This is true for both freshwater and marine tanks.


Here is a fun fact: Fish poop can be a variety of colors, depending on what the fish has recently digested. This can give you insight to the health of your fish. For example, if you see a fish with reddish poo, it could mean they are eating a lot of blood worms. Fish with green poo will have eaten a lot of greens, such as green peas. And if you see a fish with white poo, it means they are malnourished.

Similarly, if you have a fish that is swimming around with a long trail of poo hanging from it, that usually means the food it is eating is too dry or that poo was compacted.

Can Fish Suffer From Constipation?

Yes, fish do get constipated once in a while. This usually occurs when your fish are given too much food or the food they eat is too hard to be thoroughly digested. Keep this in mind.

Fish that are constipated will refuse to eat and will swim slowly, almost lazily. Also, you won’t notice any fecal matter around the substrate. As mentioned earlier, some fish will have trailing feces. This stringy poop can indicate that they are suffering from constipation and are having a hard time ridding themselves of everything.

If you fish is suffering from constipation, do the following:

Quick tips to fight Constipation

  • Give them some peas to eat. Peas are rich in dietary fiber, and that can help soften up their poo. Drop one or two peas into the tank. Your fish will usually poo within 24 hours.
  • If you don’t have any peas, offer another fish-friendly fibrous food.            
  • Avoid flake foods, since those are known to cause constipation. Follow all other feeding instructions on other foods.
  • Avoid overfeeding your fish, as well. This will cause a dangerous expansion of the digestive tract and put pressure on the swim bladder.
  • Go to the vet. If you have offered up fiber and your fish still has yet to poo, you can call a vet for advice. Act quickly. The longer your fish stays constipated, the greater the risk of it getting poisoned by toxins.   

Hopefully, you will be able to get your fish to poo. If not, you can also consider the temperature of the water. Sometimes, if the tank is too cold, it will slow down your fish’s metabolism. They won’t want to eat, and thus, they won’t poop either.

Can Fish Suffer From Urinary Problems?

Sometimes fish will develop kidney or excretory system complications that prevent them from peeing. Unfortunately, there are few ways to notice this has happened aside from a lack of ammonia in the water and visible bloat or lethargy (or both).

If your fish is having trouble peeing, it can be caused by one of the following health issues:

  • Renal dropsy: This kidney infection occurs when parasites infect the kidneys. The kidneys stop working properly, the abdomen begins swelling, and the fish has trouble swimming. Goldfish are especially prone to renal dropsy.    
  • Proliferative kidney disease: Affects fish that are commercially grown, such as trout and salmon. You won’t see this much in fish that are kept as pets.
  • Carp dropsy: Most common in carp species, this is when the kidneys are infected and waste products gather in the eyes, causing them to swell.

With any kidney problem, most fish do not survive. They will become sluggish, have difficulties swimming and eating, and usually die within six months.

Is Fish Poop Good For Aquarium Plants?

Yes, for some plants. Coral reefs, for example, require warm water, sunlight, and nutrients from fish poo and pee to survive. Other water plants also enjoy the waste products, because it is a quickly way to absorb the nutrients required for growth. Two important elements in fish poo is phosphorus and nitrogen. Algae will also thrive on fish poo and pee by converting the waste into sugar.

Yet, this is one reason you want to keep your tank clean. Too much fish pee and poo present in a tank is an invitation for algae blooms to flourish. Make sure you are cleaning your tank routinely to avoid negatively impacting your fish and plants.

Cleaning Your Fish Tank of Fish Poo and Pee

cleaning fishtank

One thing every fish-keeper is going to learn how to do quickly is cleaning out the fish tank. One of the reasons you need filtration and frequent water changes is to rid the tank of detritus and urine. This is especially necessary because fish feces is full of fungi, bacteria, and parasites that will cause illnesses if left uncleaned.

Usually, fish poop will settle on the floor of the tank among the substrate. It can also cover the leaves of live aquarium plants or settle on decorations. This is why having a powerful enough filter is important, because the filters can help clean some of the muck. However, too much filtration can create an unforgiving environment for some fish.

That is why you, too, have a duty to be done. Here is how to clean your fish tank correctly:

FishTank Cleaning guide, Step by Step

Step 1: Shut off all electrical components

Turn off the filter, heater, and anything else connected to electricity. Shut down the air pump, as well, if you have one. During this time, you can start using an algae scraper on the walls of the tank where the filter and heater used to be. Sometimes, detritus will cling to algae.


Step 2: Take out some water

Next, remove about 40-50 percent of the water from the fish tank. You can use a gravel vacuum to pull water and put it in a spare bucket. If you plan on removing the substrate to clean that, it is recommended that you take this time to relocate your fish to another spare tank for the time being, since you wouldn’t want to injure them.


Step 3: Clean and disinfect decorations and equipment

Once you have removed your fish and the water, it is time to clean the decorations and equipment. Use an aquarium-safe, non-toxic cleaner for the decorations. Scrub everything down, rinse it off, and let it dry before returning it to the tank. Rinse off the plants, if you have them, as well.


Step 4: Prepare some replacement water

Keep in mind that most experts recommend replacing 15 percent of the water in the tank once in a while. Once you do a full cleaning, replace 50 percent. If you are using tap water, you might want to boil it first to get rid of any contaminants. Mix some of the new water in with your fishes so they can get used to it.


Step 5: Wipe down the tank walls

Make sure you have wiped down the tank walls, rinsed the substrate, and have everything nice and clean before putting the tank back to normal.


Step 6: Return your fish and aquarium plants to the tank

From there, you return your fish and aquarium plants back to the tank as gently as possible. Remember to clean the tank regularly.


Hopefully the question of how do fish poop and pee has been answered for you. Now, you should be able to fill others in on the process, too. While we don’t recommend busting out this information at the next cocktail party, you can use what you’ve learned about fish poo and pee to better the health of the fish in your aquarium. By understanding a little more about fish, you can create an environment that helps them thrive, not just survive.

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