Bettas normally swim actively in captivity. Sometimes, though, bettas may hang motionless in the water, and that's not a problem. Lying down in an aquarium, on the converse, is not a normal behavior for betta fish. Bettas lying on their sides or exhibiting other lethargic behaviors indicate poor water quality and stress, and require your immediate attention.
Normal Betta Behavior
A normal, healthy betta will swim around actively. A betta may occasionally rest, either hanging in the water column or by resting on plants or other aquarium decorations. A healthy betta will always orient with his dorsal fin upward. Additionally, a healthy betta will usually have fin rays extended, revealing the entire fins. Betta enthusiasts call this posture flaring.
Signs of Poor Health
An unhappy or unhealthy betta has different body language from healthy fish. A stressed betta will tend to hide or sulk in an aquarium, and such behavior can include lying side. Bettas in a stressed state may clamp their fins shut, holding them close to the body. Unhealthy bettas may have parasites, which look like discoloration, bumps or velvety dusting on the skin. Much of the time, these problems tend to go hand-in-hand; you'll likely see more than one sign of trouble at a time.
Poor Water Quality
Poor water quality is a primary source of stress in fish. Ammonia, a waste product of fish, and related nitrogen compounds like nitrite can poison fish even in low quantities. Bettas have a greater resistance to polluted water than most fish, since bettas have organs that allow them to breathe air if they need to. Still, bettas are not invincible, and poor water quality can stress them. Another problem bettas may face is cold water. Bettas come from the tropics and need water between 77 and 82 degrees Fahrenheit to thrive. Unheated bowls and vases generally stay too cold for bettas, which promotes stress and lethargic, sideways comportment.
Bettas in aquariums tend to have less stress and health problems than bettas in bowls or vases. A real aquarium's heated, filtered water are better for betta health. According to Tropical Fish Hobbyist magazine, bettas in real aquariums usually live on average 7 to 8 years, while bettas in bowls rarely live past 2 years. In aquariums, you need to avoid strong currents and fin-nipping tank mates.