Melissa That sounds very odd, especially in a short period of time.
Fog seriously robbing watercourses of oxygen (especially so quickly) also seems sketchy, if not doubtful (IMO). Water near the surface (lakes and rivers) normally has more oxygen than in deeper locations because of bacterial action. Cold water also tends to hold more oxygen than warm water. If ice formation robbed water of that much oxygen (and so quickly), few fish would survive freeze up in northern rivers and lakes...which they do. Also, fish move to other locations where oxygen levels drop, rather than hanging around for death.