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.
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.
Do fish float when they die? it depends upon their stage of decomposition. some fish will sink and stay at the bottom of the water level until their stomach gasses/contents have released during certain stages of decomposition, then allowing it to become buoyant and float to the top.
That said, the decomposition process can sometimes result in enough gas being produced and trapped inside the fish to cause it to float to the surface. So in short, fish float upside when they die because many of them are top heavy and posses an organ in their lower region that is filled with air.
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.
If fish are experiencing acute stress (i.e., gasping up at the surface, lying on the bottom and not moving, or darting around the aquarium), you can be pretty sure that the water has been poisoned in some way. Maybe cleaning sprays got into the aquarium, or something released toxins into the water.
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.
Look at the eye as a whole. If they're sunken, your fish is dead or near death. Look for cloudy pupils, which is also a sign of death in most aquarium fish. If your fish is a pufferfish, walleye, rabbit fish, or scorpionfish, occasional eye cloudiness might actually be normal.
Place the goldfish in a container filled with cool water from his tank. The cool water contains oxygen and will help to revive your fish. Some experts also suggest placing your goldfish right back into the water in his tank, even if he appears dried out.
Check the Ammonia Levels in Your Aquarium Water
With a small fish decomposing in a 30-gallon aquarium, it will likely take around 24 hours before the water smells bad. In contrast, a smaller tank, such as a 10-gallon aquarium will become contaminated in only several hours.
If you've noticed your fish struggling to stay afloat, sinking or even swimming upside down, it's likely that your fish is suffering from swim bladder disorder.
Poor water conditions
Whenever the tank has bad water or water not suitable for the fish's health, it leads to their death. Poor water condition is a big reason why most of the fish die in a fish tank. Therefore, if you are an aquarist, the priority should always be to maintain the water quality.
If you experience irritation, get out of the water and thoroughly wash off. Do not swim among dead fish because they can be associated with harmful bacteria.
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.
Cycling with dead fish
By placing a small dead fish or prawn into your set up, you introduce ammonia for the bacteria to feed from. Doing this, you can see why it is a good idea to monitor fish health, as dead fish will need to be removed immediately, as they cause ammonia spikes.
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.
The Fish Are Suffering From Temperature Shock
If the water temperature is too hot or too cold, the fish could suffer from temperature shock. That can sometimes cause the fish to become extremely lethargic and spend most of their time lying on the tank bottom.
In our sad case at home, the energy was coming from the electric pump, that was pushing the water towards the fish. There's no magic, or even fancy fish physiology. All that swimming by the dead fish is powered entirely by the energy in the moving water.
The general rule of thumb is that if a fish has been properly handled and stored, you can safely eat it up to three days after it has died. This time frame also applies to cooking it or freezing it for later consumption.
Photo caption for Benjamin Franklin pic: An old adage attributed to Benjamin Franklin — “guests, like fish, begin to smell after three days” — has never been taken more seriously than in modern hospitality sensibility.
Benjamin Franklin famously said that guests, like fish, begin to smell after three days. Many of us are inclined to agree.
A fishy smell can be caused by some elements like Barium or Cadmium metals. Chloramine is a compound of chlorine and ammonia used to disinfect public water sources. Unfortunately, it can cause an unwanted fishy odor in your water.