Male betta fish are known for their colorful, flowing fins and for being easy-care, relatively hardy fish. Very responsive to the sights that surround him, a betta will often swim excitedly when you pass his tank. Not moving is uncharacteristic. It can be worrisome to notice that your betta has stopped moving. But it doesn't necessarily mean he is on the brink of death.
Swim Bladder Problems
The betta has a flexible organ that fills with gas and controls buoyancy. When the organ is compromised, such as when a fish becomes constipated, it can cause the betta to float irregularly or sink to the bottom of the tank. A betta who does not appear to be moving can be experiencing swim bladder problems, which makes for an intense and exhausting struggle each time he attempts to swim to the water's surface. If you suspect your betta is constipated, you can try fasting him for a few days to see if the impaction breaks up, or you can feed him one shelled pea to see if that helps.
Resting or Sleeping
Like all living creatures, betta fish rest and even sleep. According to Betta Fish Facts, bettas are tropical freshwater species who sleep when it's dark. A betta who is not moving may simply be taking a rest or sleeping. The preferred way in which they sleep can vary from fish to fish, but some bettas lay down on their sides without moving. If you notice that your immobile betta fish seems to startle and swim quickly when you turn on the lights, he may have just been sleeping.
Poor Water Quality
Bettas are sensitive to water parameters and temperature. The ideal temperature for a betta is between 72 degrees and 82 degrees Fahrenheit, according to the Aquarium Guys website. Water that is too cold can make your betta become sluggish and stop moving. Cold water can compromise your betta's immune system and make him more susceptible to a variety of illnesses.
Dead in the Water
Betta fish live for 3 to 5 years on average, but some can live a few years longer than that. If a betta is not moving, whether he appears to be floating on the water's surface or lying on the bottom of his tank, the possibility that the fish has died is entirely real. To determine if death has indeed taken place, you can start by taking a close look to see if your betta's gills are moving. Tapping on the glass and swishing the water around should at least invoke a subtle response even from a sick betta.