This depends on many factors.
Clean water of course! Dirty water is full of pollution, and makes it harder for the fish to breath. Also, dirty water that contains high amounts of ammonia, nitrite, and/or nitrate can actually cause long term gill damage in goldfish.
Yes. As long as there is sufficient heat water can evaporate in the dark.
This is not true.
Epsom salt will never evaporate. If the Epsom salts are dissolved in water, the water will evaporate, leaving the dry salts covering the inside of the container.
100 degrees
This will depend on how much vinegar you are looking to have evaporate. Vinegar is a mixture of acetic acid and water. Acetic acid will evaporate faster than water at a rate of .97.
as long as you could in a sewer drain It is cruel and selfish not to clean a tank besides it doesn't take that long get alge eaters to clean up all of that and get a tank filter to help with the mess so it isn't a lot of work.
Put 2 cups outside on a sunny day. One with salt water and one with regular water. Check on them every 5 minutes or just watch them. Then you time how long the salt water took to evaporate compared against how long it took the regular water to evaporate. Then you'll have your answer.
This depends on many factors.
This depends on many factors.
This depends on many factors.
This depends on many factors.