Where a Fish gets its oxygen?
Fish use their gills to filter oxygen from the water. However, if the water is stagnant enough they are unable to get oxygen and suffocate(this is why most fish can't live in puddles or small ponds). Some fish have a labyrinth organ that basically acts as a human lung, so they can breathe both air and underwater and don't have to worry about this(such as betta fish).
Different species, different temperature requirements. As water gets warmer the oxygen level reduces and the fishes metabolism gets faster. The fish now requires more oxygen out of water that has less. The equation eventually stops when the fish can pant only so fast and eventually dies/expires through lack of oxygen or exhaustion.
Oxygen that is dissolved in the water crossed ghe gill menbrane by diffusion. The oxygen is dissolved in the water like carbon dioxide in a beer. This oxygen is not a part of the water molecule. The oxygen in the water molecule is not available to the fish, when dosolved oxygen gets low, yhou have a fish kill.
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.
Why is it possible to grow plants in a closed glass bottle but a goldfish bowl must be open to for the goldfish to survive?
Fish still need oxygen to live and they get their oxygen in the water. Every bubble is filled with oxygen. A fish that lives in the ocean will get oxygen from waves crashing and making oxygen, the oxygen stays in the water until a fish uses it. The fish filters the oxygen in through it's gills, leaving the water outside it's body.
Fish need oxygen to live, just like humans. But while humans use their lungs to take in oxygen from the air, fish are designed to take oxygen from water. The water passes through a fish's mouth and over a set of gills, which absorb oxygen. Without lungs, a fish out of water is a fish without oxygen.
Fertilizer IS nutrient rich. When it gets into lakes and ponds, it causes massive algae blooms. These massive algae blooms soon die off, contributing to massive amounts of carbon dioxide accumulating in the lake or pond (and thus a decrease in oxygen concentration). Such a decline in oxygen concentration will suffocate fish and thus cause fish kills.
No fish are supposed to go in a fish tank, not even gold fish, there's no oxygen for them to breathe, and its too small, which gets dirty really fast, so evidently, buy at least a 1 gallon tank with a filter, so you'll clean it less, fish can live longer, and there will be plenty of room to swim in.