The fish gets its oxygen from the water, from the limited amount dissolved there. Unless more oxygen is added to the water, the fish will "drown" (asphyxiate). In goldfish bowls, it is important to trickle in some water regularly to encourage oxygen intake while also balancing evaporation. In larger aquariums, or with more fish, an aerator bubbles air through the water to add oxygen.
The pollutants react with the oxygen dissolved This means there's no oxygen available for the fish. That kills more fish than the actual toxicity of the pollution in many cases.
Before a storm, there is very little dissolved oxygen in the water, so fish rise to the surface for oxygen,which is more plentiful near the surface.
It all depends on the amount of oxygen in the water. If the water is low on oxygen then a fish must work harder to get the oxygen it needs so it will flap it's gill more often.
They cannot. Usually you want 1 gallon of water per square inch of fish. Cooler water actually holds more oxygen.
To incease the surface area of the gills so more oxygen can be diffused from the water the fish swims in into there bloodstream where the oxygen can be carried the fish's cells for cellular respiration
Goldfish breathe slower in cold water because there is more oxygen in cold water. Therefore they don't have to breathe as much to get the amount of oxygen they need
Some types of fish do live in acidic water. Water with a PH level below 7.0 is considered acidic water. The lower the PH the more acidic the water, the more acidic the water the more aluminum is released which is unsafe for fish. Also, the more acidic the water the lower the dissolved oxygen content is. So having water too acidic for the type of fish that inhabits the water is keeping the fish deprived of proper oxygen levels which results in lower body weight, stunted growth, lethargic behavior, fish eggs being unable to hatch, shorter life spans and death.
Hypoxia is a deficiency of oxygen in either the fish or water itself. Water naturally has a level of oxygen dissolved in itself. That is the oxygen that fish take in through the gills. This level of oxygen in clean pure water is about 8 ppm. In the summer more algae grows in the water due to favorable conditions. This algae consumes oxygen from the water. Also, in the summer the water temperature increases. Oxygen has a lower solubility in water as the temperature increases. Therefore the level of oxygen in the water will decrease slightly as the water temperature increases. These are just a few common reasons. Every water body is certainly different.
No, they shouldn't. This is of course you are doing it properly. I have heard a story of someone actually blowing into the water with a straw thinking they are putting oxygen into to water. This is just going to stress the fish which could kill it.
This difference in energy expenditure is due to the different ways in which mammals and fish extract oxygen from their environment. Mammals have more efficient respiratory systems, allowing them to extract more oxygen per breath compared to fish, which require continuous movement of water over their gills to extract oxygen. The process of pumping water over gills is more energetically demanding, leading to the higher energy expenditure in fish.
Oxygen in water and oxygen in the air have different levels of availability for organisms to use. Oxygen in water is more readily available to aquatic organisms since it dissolves in water more easily. However, terrestrial organisms, like humans, primarily rely on oxygen in the air for respiration.