I've got fluoride in my water supply, and have had only two fish deaths that I'm not sure what caused them. One of them was a cory who was alone in a community tank. The other was a
betta who seemed to be too small for his fins.
The fluoride thing is a touchy subject. Studies are showing that folks who drink mostly bottled water are having more cavities, so it's pretty likely that the fluoridated water sources are helping tooth health. However, fluoride can cause harm to the nervous system in large enough amounts (my brother has familial tremors, and there was concern that fluoride was making these worse because he doesn't spit his toothpaste or mouthwash out. A fluoride-free month disproved this, but it was a legitimate concern).