Fish dont swim after water change?

Your fish are most likely stressed after the water change. Having a person work in the aquarium can make them nervous and lie on the bottom. The white minerals on your filter tubes are most likely limescale from hard water. It is harmless. Just sc it off when it gets too corroded. But don't let it fall into the aquarium when doing so. It will raise the mineral content of the water. Do it a day before a water change, That way you'll be siphoning out the excess minerals that might dissolve from any flakes that fall in.