The density of a fish is much smaller than the density of water, therefore the fish does not sink, but it should float. The only thing that keeps the fish from floating to the surface are their swim bladders. They are organs inside the fish that contain gas to balance out the density of the fish and make the fish achieve neutral buoyancy and stay in the water.
The new water is probably wrong for them. If they're suddenly in water with a different temperature or chemistry they feel very uncomfortable and stressed out. If it's too big of a shock or if you forgot to add the water conditioner it might kill them. It's best to only change some of the water frequently instead of all the water at once so it stays mostly the same.
This is just a guess, but I think that the fish slows in speed because it has used a lot of its energy to change temperature/adapt to temperature change, so has less energy to move quickly.
they have to adjust to the temprature
Unfortunately, I believe it's time to replace the fish.
environment and topography
By moving water.
put the live fish in a bag filled with water and change the tank then put the fish back in.
by wearing away small pieces of land and carrying them to new places
yes bubbles help bring air to the fish and keep the water moving
They are used to propel the fish through the water, and in some cases they are used to keep water moving through the gills!
Moving water
well if its cold water fish then 3-4 days :)..
When you are changing the water in a fish bowl, you have to empty cup ot bowl with water in it. Then, put it into clean water.
It depends on if you are doing a complete water change or a partial water change. I usually just leave the fish in and siphon off about half of the water than replace the water. The fish get a bit nervous but seem to be fine with being left in the tank. To answer your question: No, you do not have to remove the fish when changing the water.
Not successfully. Moving water will likely knock the eggs out of the nest. The water should be near still with only an airstone present.