Okay I have three Suggestions.
Hexamita (Hole in the head)
Symptoms: What appear to be small holes in head, ulceration of lateral line.
Another name for this disease is "Hole-in-the-Head" disease. Even though this is a disease of the digestive tract, it often manifests itself as ulcerations in the head and/or lateral line. Discus and other large cichlids, especially Oscars, are especially prone to Hexamita. As it is a disease of the digestive tract, a wasting away or loss of appetite may be experienced.
An effective treatment is the drug metronidazole. A combined treatment in the food (1% in any food the fish will eat) and in the water (12 mg per liter) is recommended. Repeat the water treatment every other day for three treatments. Improvement in aquarium conditions has helped in the disease just going away with no treatment.
Now although the above doesn't really seem to apply, i have heard of this disease causing such effects as mentioned by you.
Ichthyosporidium
Symptoms: Sluggishness, loss of balance, hollow belly, external cysts and sores.
Ichthyosporidium is a fungus, but it manifests itself internally. It primarily attacks the liver and kidneys, but it spreads everywhere else. The symptoms vary. The fish may become sluggish, lose balance, show hollow bellies, and eventually show external cysts or sores. By then it is usually too late for the fish.
Treatment is difficult. Phenoxethol added to food as a 1% solution may be effective. Chloromycetin added to the food has also been effective. But both of these treatments, if not watched with caution, could pose a risk to your fish. It is best, if diagnosed soon enough, to destroy the effected fish before the disease can spread.
Swim-bladder Disease
Symptoms: Abnormal swimming pattern, difficulty maintaining equilibrium.
Swim bladder problems usually indicate another problem. If you suspect swim-bladder problems in a fish, first check and treat it for other diseases such as congenitally deformed bladder and Cancer or tuberculosis in organs that directly affect the swim bladder.
As for the Fish that Died Yesterday, that could be a range of diseases, most likely would be Flukes, but Velvet and White spot, also invade and inflame or agitate the Gills
Shae