Do Fish Sink Or Float When They Die? [Explained]

It happened to me a few times that all of a sudden I notice one of my fish is missing. As a general understanding, I thought dead fish must float. But I didn’t find the dead fish’s body. So, do fish sink or float when they die? Probably, you’re looking for the same exact answer about your missing fish.

In general, most fish sink to the bottom immediately after death. However, over time when the fish’s body decomposes, gases fill up the body cavity. As a result, it becomes lighter than the water and floats. However, not all of the dead fish float, and sometimes fish bodies may decay at the bottom unnoticed.

In the below sections, I’ll discuss more on the topic such as when the fish sink or float, how often it takes for the dead fish to float, and some other relevant information. Stay tuned!

Why Do Fish Sink To The Bottom After Death?

Fish contains an organ called the “swim bladder”, similar to the human lungs. They use this organ to sink or rise to the different depths of the water body. Fish fill in the swim bladder with oxygen to rise to the surface and deflates to reach the bottom.

To stay at a particular level, a fish fills its bladder to the point at which it displaces a volume of water that weighs what the fish weighs. In this case, the forces of buoyancy and gravity cancel each other out, and the fish stays at that level.


When the fish is no longer alive, the swim bladder becomes empty of oxygen. As it deflates, fish sink to the bottom. However, there is some exception.

Fish may float as soon as it dies if the fish was affected by swim bladder disease. Swim bladder disease disrupts the hydrostatic balance of the fish.

They find it difficult to control their ability to sink or float. If the fish dies due to swim bladder disease there is a possibility that it may float immediately.

They can also float instantly after death if the fish had a bloated stomach before death. Gas remains in the stomach making the fish’s body lighter than the water. As a result, fish float.

Why is my fish dead but not floating?

In many instances, your fish may not float after death, and it’s very common. Let’s see, what are the most common reasons?

Trapped Inside: If you have aquariums with rocks, decors, or plants, there is a good chance that the fish get trapped inside. If the fish is not large enough, or you have a big tank with lots of other fish, it’s very common. I faced the same situation in one of my 100-gallon community tanks when I find out the fish body was tangled inside a narrow space around decors.

Jumped Out: Some fish such as betta have the tendency to jump. If you don’t tank lid or cover, and if the water height is high enough for the fish to jump out of the aquarium, then probably you can consider this as well. Search for the fish at the close periphery of the tank, and to prevent that from happening again use a tank lid or cover or keep the water level at least 2” low from the top of the aquarium.

Hiding Out: Sometimes you may think that fish dies in your tank as you haven’t seen them for a long time. Fish like to roam around the tank, and often enjoy their privacy. So, if the fish is not showing up, probably it’s hiding out. Shrimps, snails, and other shy fish tend to do that frequently. So, search well inside the tank.

Decomposes Completely: The last thing that can happen is the fish’s body decomposes entirely while it was trapped, and the skeleton is also out of sight.

As the fish sometimes may never float, try to maintain a headcount every other week, so that you can know as soon as you have a missing fish.

How long does it take for a dead fish to float?

Dead fish that sinks to the bottom floats after 5-10 days depending on the tank condition. In a cold water tank, it may require 8-10 days, and in a tropical aquarium, fish may float within two days.

Temperature plays an active role in the process of decomposition of the fish body. The higher the temperature of the water the faster will be the decay rate.

As I said earlier, if the fish has trapped gas inside, it will float sooner. However, fish stuck in the crevices, decors, or plants may never float. I have had such instances when one of my fish died, and it never floats but rather decomposed at the bottom.

It can also happen even if the fish wasn’t stuck in any place but just decayed inside the aquarium or was eaten by other members of the tank.

how long does it take for a dead fish to decompose

The decomposition process starts after 3-4 days of fish death, and for the whole fish body to disappear it may take 7-10 days. However, the number of days may vary a bit depending on other conditions.

If the fish body decomposes in your fish tank, you’ll often experience ammonia or nitrite spike. So, whenever you have a missing fish, try to test the water parameters, and take necessary actions so that the decomposing fish body can’t do harm to the other inhabitants of your aquarium.

How do you know when a fish is about to die?

Fish show different signs, and if you’re experienced enough you can understand those. A dying fish may show symptoms like weakness, hiding more, labored breathing, lack of appetite, and so on.

A certain disease-affected fish will show symptoms of that specific illness. For example, fish suffering from dropsy, swim bladder diseases may find it difficult to control their position, and often sinks to the bottom.

Fish scales and skin also have some visible signs if the fish is injured or sick, and going to die. The most vital sign is no movement for a longer period of time.

I have a personal experience of losing one of my Zebra Danios showing this symptom. The fish often stayed at the bottom for a long time as if it was dead. However, after a while, it started moving, and then again plunged to the bottom unmoved. It happens a few times before it died in 2-3 days.

After that, I learned a lot about saving sick fish, and I have written an article on that topic. I encourage you to read that one to revive a dying fish.

More Resources:

Bottom Line

Fish may either sink or float, depending on the conditions prevailing before death. However, most often the fish sink at the bottom first and after a few days, it floats.

Sometimes you may never see the dead fish’s body. The reason is that it was consumed by other aquatic critters, entangled in narrow crevices, or decomposed in the aquarium unseen.

Feel free to ask any question in the comment section below that arises in your mind while going through this article. I’ll answer your queries within the shortest possible time.

Sujit Modak

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