As far as I know the washed out appearance is caused by stress. Stress can be caused by a number of factors one of which is less than suitable water chemistry including pH levels, most fish will aclimatize to a certain changes in pH after a time but the key is to keep pH levels stable i.e not fluctuating. Certain fish such as discus can be intolerant to higher pH levels wheras others can stand a wide range.
The question is, what type of fish are you enquiring about and do you have any objects in your tank that would alter the pH of your tap water.
For info other forms of stress could be boisterous tankmates, unsuitable light levels, inadequate areation, poor diet etc,etc,etc