A heater can raise the temperature of a fish tank if needed
Yes you can, but typically they are mesured in gallons
no because if its a freshwater tank the rain has salt in it
No, fish are not vaccinated. If you do buy a new fish, you should put them in a separate tank first before adding them to your main tank and treat them for possible parasites first; you can use a product such as Prazi for treatment. This should be done any time you have a new fish that you want to add to your established tank as it's easier to treat a 10 or 20 gallon tank with 1 fish in it than it is to treat a much larger tank with several fish (as one new fish can make your entire tank sick)
depends on the size of the fish, there might not be enough oxigen in a small fish tank.
It is not recommended to use a tank that previously housed a snake for fish. Snakes may carry diseases that can be harmful to fish, even after thorough cleaning. It's better to err on the side of caution and use a new tank for fish.
The simple answer is yes.
math
Of course you can.
It will either have to get used to it or die.
no
No