Why does the water smell?

I rescued a betta from walmart he was in brown water. I put him in a betta bowl and bought items like bamboo and an leaf that sticks to the side for him to rest on I did a water change (by the way I have well water) and I have had him about a week and I woke up and saw his poor little fins ae all frayed and the water had a horrible smell to it so I did a water change of about 70 percent I was wondering what this could be or if there is something I should do