All fish vary in their dissolved oxygen needs. Oxygen levels are measured in parts per million (ppm). When water is fully saturated with oxygen, dissolved oxygen level is about 10ppm, depending of course on water temperature. It may be noted that fish are much easier to keep in your pond when the water is cold. Fish in cold water require less oxygen and the water can hold more oxygen.
Oxygen gets into water through the contact of air and photosynthesis by aquatic plants. It is then consumed by respiration of aquatic organisms like bacteria and fish.
In fertile ponds or lakes, organic matter is so plentiful that oxygen is consumed faster than it is replaced. In the summer time often the pond or lake, stratifies, and the layers of decaying organic ooze on the bottom rapidly consumes oxygen and with no circulation to replace it, oxygen levels wane, forcing the fish into shallower areas of water.
The fall weather replenishes oxygen levels, but they will dip again in winter. If the snow and ice covers the pond or lake enough to prevent sunlight penetration, plants cannot produce oxygen through the process of photosynthesis, so the amount of oxygen plummets, and if it plummets to far, the fish will begin to die. Dissolved oxygen is replenished in the water by oxygens contact with air. Wind circulates the water, carrying oxygen to the fish in the depths. In the summer months the lower layer does not circulate, so no oxygen is added. However, when the leaves of plants are exposed to sunlight, oxygen is produced through photosynthesis. Again though when the sunlight cannot reach the plants, no oxygen is produced.
Although, all fish are cold-blooded, the different species prefer different water-temperatures. Examples: coldwater fish like trout or salmon like water temperatures in the fifty degree range. Other fish like pike and perch, like sixty degree temperature, and warmwater fish like bass and sunfish, and catfish like water temperature in the seventy to eightly, degree range. However, all of these types of fish, as well as, many others need food and will leave their temperature ranges to find food.
So in summer when the oxygen levels drop in deep water, fish go into shallow water where oxygen levels are higher. Most warm water fish can tolerate very cold water. I have seen this in my brother's fish pond where due to a leak his pond water level dropped and his koi were swimming in much shallower water than normal. In fact there fins were sticking up out of the water. But in this shallow water they remained healthy. Luckly, he was able to repair the leak and bring the water levels up to normal levels. In Florida, we had a very large pond and our koi preferred the deepest areas of the pond during the summer, and could only be brought to the surface by feeding them.
