Why density at different depths change?
Because of the weight (pressure) of all the stuff above pressing down. For solids this results in hardly any change in volume, and for liquids just the same. But for gases, such as the atmosphere, it has a large effect so that at 100miles or so above the earth, air prsssure and therefore density is practically zero.
The density of a rock doesn't change when it is submerged in water does your density change when you are submerged in water?
How does the density of gases at different elevations in the atmosphere differ from the density of liquids at different depths?
Mass doesn't change density because density is a qualitative property, meaning it is a quality for material's density, not a measure of how much density in the material. A quality is the same thing as a trait, for instance, pine trees have pine needles, even the little ones, it does not change because they are different sizes because it is a trait for a pine tree to have needles.
Just a stab here, but I belive how fish do it... is that they take the air out of the water they are in. They then store this air inside their swim bladder. This stored air helps counteract their weight. They balance the amount of air compared to the density of the water, and the amount of water they have pushing down on them. This allows them to hover at different depths. Mostly, their fins…