To tell if a fish is resting or dead, check for gill movement (slow for resting, none for dead), eye clarity (clear for resting, cloudy/sunken for dead), and responsiveness to touch (slight wiggle vs. stiff/unresponsive), noting resting fish often stay still in specific spots or hover, while dead fish are completely motionless, often sinking and stiff, with sunken/cloudy eyes and no gill action. A dead fish will also have no reaction to being gently scooped with a net, unlike a sleeping one that wakes up.
How To Tell if a Fish Is Sleeping
It sounds like your fish is showing signs of slowing down, which can happen as they age, but staying at the bottom or moving very little can also point to stress, illness, or internal issues like egg binding, swim bladder problems, or organ failure.
If you find your fish struggling to swim, trying to swim only at the surface, or sinking to the bottom, it could be because of swim bladder disease. The common causes of the disease are poor water quality, overeating, digestive issues, kidney malfunction, and secondary bacterial infections.
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.
Checking Your Fish's Vital Signs
If your fish is just sleeping, they'll wake up and try to wiggle their way out of the net. If they don't, they could be dead or very sick. Look for breathing. Check the gills for most species.
While fish do not sleep in the same way that land mammals sleep, most fish do rest. Research shows that fish may reduce their activity and metabolism while remaining alert to danger. 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 are some of the most common signs of disease seen in pet fish:
In general, fish need darkness in order to rest, especially since they have no eyelids to cut out the light themselves. I keep the light under my microwave on throughout the night, but it's not a bother to the tanks in my kitchen. Sounds like yours have learned where to go when they need to sleep.
Erratic swimming, hiding, or a lack of appetite are all signs your fish may be stressed.
This can lead to aggressive behavior or a decrease in their quality of life. Potential for Injury: If the fish is startled or stressed, it could injure itself by swimming erratically or bumping into the glass or decorations inside the tank.
It's normal to do a water change after finding a dead fish. Unfortuneally the cycle of life is a big part of keeping fish.
At first they usually sink to the bottom as gravity takes over, and then as they start to decompose gas builds up and causes them to float. Then after a while the tissue breaks down sufficiently that the gas isn't trapped anymore and they sink back down to the bottom.
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.
Can a Dead Fish Be Revived?
Cichlids have evolved into thousands of different species far quicker than it took apes to evolve into just three species. This fish plays dead in order to attract other fish then eats the fish who come and investigate.
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.
A sudden color change—fading hues or dark bands—can be a sign of stress. If your fish is shimmying with clamped fins near the surface, it's not just dancing—it's a red flag. Erratic swimming or 'porpoising' could mean trouble with ammonia or nitrite levels.
Fish affected by swimbladder issues struggle to stay buoyant. They typically look well but float near the water's surface, or sink to the bottom, wobbling about on their side or upside down. They can get exhausted trying to stay upright, or trying to feed.