When the bladder is completely deflated, the fish has minimum volume and sinks to the ocean floor. 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.
The faster they swim, the more water is pushed through their gills. If they stop swimming, they stop receiving oxygen. They move or die.
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.
This can be due to overeating, overconsumption, low water temperatures, bacterial infections, parasites, or other impaired organs affecting the bladder. In these particular cases, the fish can end up with a distended belly, curved back, impaired swimming, or even death leaving them floating on top of the water.
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.
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.
Weakness or listlessness. Loss of balance or buoyancy control, floating upside down, or 'sitting' on the tank floor (most fish are normally only slightly negatively-buoyant and it takes little effort to maintain position in the water column) Erratic/spiral swimming or shimmying.
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.
The best way to find that your fish is sleeping or dead is by moving them. Try to scoop your fish into the fishnet, if they are sleeping, they will suddenly be active and try to wriggle out of the net, and if the fish is doing no movement, then they are dead or in critical condition.
Really efficient fish species don't even need to swim around much to draw the oxygenated water across their gills. Other species, such as tuna, have to keep moving -- and keep their mouths open -- to keep the process going.
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.
Sinking/Floating to the Top
If the swim bladder is deflated, it will sink in the tank. If the fish has gulped in too much air while feeding, this may cause it to float to the top of the tank.
As mentioned, most people think that fish sink to the bottom or float on the water's surface when they die. But in reality, there is a discrepancy between floating and sinking depending on the cause of death.
Sadly, a dead fish cannot be revived. However, all is not lost if you have lifeless fillets aging somewhere in your freezer since last summer. These fish are losing their table-quality as each day passes.
dead fish (plural dead fish or dead fishes) (slang) A sexual partner who lies flat and unresponsive during sex.
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.
If your fish is swimming frantically without going anywhere, crashing at the bottom of his tank, rubbing himself on gravel or rocks, or locking his fins at his side, he may be experiencing significant stress. Talk to your veterinarian about treatment and look into what may be causing the stress and alleviating it.
They could also be bottom sitting due to water conditions, parasites or bacteria, stress, or gastrointestinal problems. If they are bottom sitting due to water conditions, then this needs to be corrected with checking the water daily and doing large water changes until the water is within appropriate parameters.
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.
However, they may still be at risk of developing similar problems. Fortunately, swim bladder problems can usually be cured fairly easily, as long as you spot them early enough. If you see your fish swimming upside or on its side, act quickly! There are several good swim bladder treatments available.
If a fish has a bent or curved spine, it is most likely infected with a Gram-positive mycobacteria (Mycobacterium marinum or M. fortuitum). This is commonly referred to as fish tuberculosis, piscine tuberculosis, acid-fast disease or granuloma disease.
Most fish rise and sink in the water the same way a helium-filled balloon or a hot air balloon rises and sinks in the air. Buoyancy is caused by a difference in fluid pressure at different levels in the fluid. Particles at the lower levels are pushed down by the weight of all the particles above them.
Fish will not get tired in a tank under normal situations.
The energy for swimming is stored in their muscles. They also do rest and “sleep" in between. They don't have eyelids, so they sleep with eyes wide open and can see even when they sleep.