you never know if a fish is sick or not just by looking at him. the common problems, such as ich that you've encountered, are easily noticeable, other problems such as parasites take some time to be noticed. the fact that your fish is stuck on the filter means that it didn't have the strength to swim away from the intake, a clear sign of some weakness in the fish. Healthy fish are used to swim upstream and what-not in their natural environment, especially during breeding seasons. I would bet my money on the fact that the fish was sick before it got stuck, it's just that the symptom was not apparent.