Is the fishes abdomen swollen and do the fins look like they are sticking up?
Mycobacterium or fish tuberculosis is a bad bugger and can cause harm in humans. The typical symptoms are weight loss, the fish looking thin behind the the head and abdominal area with the subsequent swelling of the area behind the pectoral fins where the liver an other organs are. The eyes will also show signs of popeye. Fish TB is largely from infected fish being introduced into the tank or from being fed raw fish.
Cornebacterium or pseudomonas is similar but without the thinning on the area behind the head and abdomen. The fish will swell up behind the pectoral fins from gas a fluid build up and the scales will lift. The protrusion of the eyes or "popeye" are also symptoms