Well you have only had the snake for 3 days so far, so there isn't too much to worry about as of right now. The snake isn't going to die in a weeks time, you have plenty of time to try and get it to eat. Yes, I wouldn't call fish a good nutritious thing to feed your conda...
First off, wait till your snake gets settled in before trying to feed it again, don't try for about another 5-6 days. There are a billion different ways to get condas to eat. Next time you feed, try putting a f/t rat pup or small rat on the water bowl, dangle it over it I mean, when the snake is soaking, and then turn off all the lights, so it's completely dark and leave it in there overnight or for a few hours and come back and see if it is gone. Keep posting on how everything goes.
Good luck!
~Ben