Hypoxia is when fish don't have enough oxygen to breathe. Just like your body needs air to breathe, fish need oxygen to live. Fish take oxygen from the water through their gills. But sometimes there isn't enough oxygen in the water for the fish to breathe.
There are a few reasons why this might happen. One reason is when there are too many fish in one area. If there are too many fish, they use up a lot of the oxygen in the water. This can make it hard for all the fish to get enough oxygen to breathe. Another reason is if the water is very warm. Warm water can't hold as much oxygen as cold water. So if the water gets too warm, there might not be enough oxygen for the fish.
When fish don't have enough oxygen to breathe, they might get sick or even die. To prevent hypoxia, it's important to make sure fish have enough space to move around and that the water is the right temperature. People who take care of fish, like fish farmers, often test the water to make sure it has enough oxygen.