There are a number of reasons why pond water would become stagnant in the summer. Pond water may not be stirred often enough.
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If your pond isn't at least 20 ft. deep, at some point. It is almost a guarantee that the pond will need chemicals to help keep it clean. Shallow water, because of heat gain and lack of circulation and aeration and runoff and the fill around the pond; even the vegetation and animals can lead to problems.
They live in stagnant ponds and rice patties in Southeast Asia.
When air temperatures heat stagnant water, such as in ponds and lakes, the dissolved oxygen levels are reduced. The fish cannot get oxygen out of the water via their gills and they suffocate.
Stagnant water can be found in areas where water accumulates and remains still, such as in puddles, ponds, or unmaintained water containers. Look for signs like algae growth, debris buildup, and a foul odor, which are indicators of stagnant water. Be cautious around stagnant water as it can be a breeding ground for mosquitoes and other harmful bacteria.
A stagnant ecosystem refers to an ecosystem that lacks movement or change, often characterized by a lack of diversity, low productivity, and limited or no movement of energy and nutrients. Stagnant ecosystems typically have low levels of oxygen, causing a decrease in biodiversity and overall ecosystem health. Examples of stagnant ecosystems can include stagnant ponds or lakes with low water flow or oxygen levels.
winter
Water can become stagnant and unsafe to drink within 48 hours if not properly stored or treated.
Water can become stagnant within a few days to a few weeks, depending on factors such as temperature, sunlight exposure, and the presence of organic matter.
Stagnant ponds and rice patties in the Vietnam area. They breathe air because the water doesn't have any oxygen in it.
Since goldfish are bred in captivity, there aren't a lot of wild goldfish. Goldfish live in lakes and ponds and have no need to migrate. During winter they hibernate in the frozen water. They are active in the Spring and Summer when they breed and grow.
The other creatures like snakes and fish eat them.