Hi Bob, Thanks for your response. Shortly after starting to notice stress in my fish, I actually did a lot of reading on the nitrogen cycle and realized that the pet store misguided me in letting me purchase so many fish before cycling was complete. Accordingly, I have been adding salt, been on reduced feeding schedule, and doing very frequent water changes to try to remedy the situation. Hence, the tank is nearly fully cycled and the fish are well (Except for the one in question). The problem I am really concerned about at this point is, if this was a parasite (perhaps a nematode, as you suggested), are there measures that I can take to prevent my other fish from dying.