Many people stock their yard ponds with goldfish or colourful koi. Both are species of carp. If you live in the north and the surface water is cold from extreme weather, they will head for the bottom to keep warm. But the major reason fish stay on the bottom of a pond is that is where the oxygen is. They need to breathe.
Fish need oxygen
When fish don't get oxygen, they suffocate. They go where they can find oxygen. Warm water on the surface of a pond contains less dissolved oxygen than the colder water on the bottom. Water that is dense, including water at the bottom, holds more dissolved oxygen. The availability of oxygen in a pond is different in the summer than the winter.
Spring and summer
In the late spring and summer, the rapidly warming water in the top 2.4 to 3 m (8 to 10 feet) of the pond loses oxygen. Even if it is colder at the bottom, the lack of light there prevents photosynthesis, which produces oxygen, and plants rotting there consume oxygen. In the summer heat, however, there is still more oxygen on the bottom than on the top. That's why the fish stay on the bottom.
Autumn and winter
In the winter, the fish will often come to the top because the water there is getting colder. Since the cold surface water holds more oxygen, it is inviting to fish. But cold water sinks. Warm water rises. Strong, frigid winter winds, plus rapid drops in temperature and the inflow of large volumes of cold water from heavy rains or snow can cause lower, warmer layers of oxygen-deficient water to rise, replacing the upper layer of water. Fish follow oxygen. If the best water is suddenly on the bottom, down they go.
Fear of a new environment or fear of predators will sometimes drive fish to the bottom of a pond. Some owners of goldfish say that their fish are initially wary and have to get used to their new home. It will take a few days for them to get settled into your pond. If you already have fish in the pond, they have to get used to sharing the water. All fish have a natural fear of predators.They can sometimes think shadows of shrubs and bushes are predators. Herons are predators feared by pond fish. Other animals that prey on pond fish are foxes, mink, owls and hawks. Once a predator strikes, the fish are twice as wary. Some pond owners keep a net over the deep end of their ponds to foil predators.
Parasites on koi
If you notice that any koi that stay at the bottom of the pond are banging their heads, they are likely infested by parasites.