FreshyFresh ....... that was most likely due to a very low disinfectant residual. Try that where I live, with any fish species of any size, and you can kiss them good bye.
Depends on the stock you keep as well. I know I've forgotten to add Prime to my goldfish 55g, that contains 4 adult goldfish after a ~75% WWC. Didn't effect them at all. If this happened with fry or sick/weak fish, it would probably effect them in a bad way.