Most fish are slightly denser than water, so sink immediately after death. However, like a drowned human, they become more buoyant over time as bacterial decomposition produces gases inside the body. Usually, enough gas builds up in body cavities to make the corpse float, like an inflated balloon.
After the fish dies there is no more DO being ingested and the air in the bladder starts to dissipate, causing the fish to sink to the bottom. After a few days, the internal organs of the dead fish decompose and a gas is formed. This gas causes the fish to once again float.
Not all dead fish float and floating can depend on the decomposition process. Small fish like tetras and guppy corpses tend to float whereas catfish and cichlids tend to sink. A pond fish may sink first then float later as gases build up in the body, and that's when you'll spot it.
Positive buoyancy disorder, where the fish floats at the surface or on its side, is the most commonly presented form of swim bladder disease, especially in goldfish. Most likely, the cause is overinflation of the swim bladder. Overinflation usually takes place in the posterior chamber.
It's pretty easy to tell when fish are sleeping: they lie motionless, often at the bottom or near the surface of the water. They are slow to respond to things going on around them, or may not respond at all (see some sleeping catfish here). If you watch their gills, you'll notice they're breathing very slowly.
Very fresh fish still have their neurons fully intact, even though they're dead. As soon as you add a bit of salt to the exposed muscles, the neurons are triggered and the muscles contract. So very dead fish will continue to move about until they use up all their energy stores.
Depending on how deep a fish was when it died, it might not rise to the surface, at least not until decomposition really sets in. Some fish never gain sufficient buoyancy to float and decay under the water. In case you were wondering, other dead animals (including people) also float after they start to decay.
"As the fish decomposes, gases fill the body cavity." The belly becomes a guts-filled balloon and the fish floats to the surface. Most of a fish's mass is bone and muscle on its dorsal side, so as the belly balloon rises, fish tend to flip upside down.
It Could Damage Your Septic System or Plumbing
While your septic system and plumbing are designed for disposing of undesirable materials, dead fish are not on the list of flushable items. Septic systems are only meant to handle human waste, water and toilet paper.
Any dead fish should be removed, as its body will quickly rot in the warm, bacteria-laden water. A corpse will pollute water, risking the health of other fish in the tank. If it died from disease the last thing you want is other fish consuming its body parts, so remove immediately.
The Water Flow Is Too Strong
Many fish species prefer a calm water environment. If the tank water current is too strong, the fish might struggle to swim or eat comfortably, preferring instead to gravitate to the bottom of the tank to escape the water flow.
If you find your fish floating on its side, not feeding it for three or four days can often solve the problem as the fish's body recovers from the gorging and rights itself again.
All that swimming by the dead fish is powered entirely by the energy in the moving water. Of course, evolution being what it is - real - sea creatures have evolved to take advantage of getting a energy boost from the water moving around them, and not just when they're dead. Most fish have a "lateral line".
Some fish float in place, some wedge themselves into a secure spot in the mud or coral, and some even locate a suitable nest. These periods of "suspended animation" may perform the same restorative functions as sleep does in people.
As soon as a fish dies, the other fish in the tank will begin feasting on their former friend. It's not personal; it's nature. Don't assume they murdered their former tank mate. They are just taking the opportunity to have a fresh meal.
How long this state lasts depends on how it was killed and how it was treated after death. A fish that is killed properly and immediately iced down will stay in rigor mortis up to five or six days; an improperly killed fish will stay in rigor mortis only a few hours.
When reviving them, you need water to flow through a fish's gills. Here's how you can do that: Move them in a figure eight to get water flowing through their gills. Position the fish into the current so water is flowing through their gills.
Some species play dead to fool other fish, often targeting scavengers as a food source. The Central American cichlid is a perfect example. In other species, though, this behavior might hint at a health issue.
Most dying fish can be easily revived with changes to the water. Maintaining water quality is essential to keeping your fish happy and healthy—and alive. You can buy a fish tank water testing kit at most pet stores. These tests can help you identify any problems with the water, such as high ammonia.
You should do a water change after a fish dies in your tank, as this will help to minimize the risk of a spike in ammonia levels and other pollutants that can cause further harm to your fish.
Ammonia and nitrite are just two of many common problems that you might come across while caring for an aquarium and both can harm your fish. A number of things can lead to ammonia, such as fish waste, leftover food rotting at the bottom of the tank, and dead fish not being removed promptly.
Staying near the water-surface or piping ('gasping' or mouthing for air) Increased respiratory rate or laboured breathing. Flashing (scraping/scratching themselves against objects) Clamped fins (fins held flat against the body).