How do you prepare water for a fish tank?

Empty the tank and scubb it out. Replace or boil and wash any gravel, rocks or other decorations. Replace all plants with new ones. Then follow the basic rules for keeping any fish successfully, which are as follows. 1 inch of fish needs at least 1 gallon of water. :- Every tank must have a permanently running cycled filter. :- Every tank must have at least 50% of its water replaced every week. If you follow the above rules and also supply the fish with the temperature and other special water parameters that its species needs, the fish will stand a chance of living a reasonable life. I can guarantee that failure in any of the above will result in your fish having constant health problems.