Thanks Ewok, I am getting a little worried because this morning I also noticed a spot of the same "fuzz" on the tip on one of the "crowns" of his tail. It's moving fast....
The Bettamax says that it also treats bacterial infections and is a 5 day treatment dosing every other day and then repeating 3 weeks later.
One of two things might have happened. A couple of weeks ago I put some ghost shrimp in with him. Some of them died (I removed them as soon as I noticed they were dead), and the rest I moved to another tank. Maybe they carried some kind of bacteria. I might also add that they are all dead now. I should have quarantined the shrimp before adding them to my tanks, but I guess I didn't think :argh
Also, the temp in the tank has been fluctuating. I don't know if it's a heater problem, or an outside temp problem. It's been cold here lately we have been keeping the thermostat turned down at night. Just to add: there is a 50w heater in this tank (6 gallon). I'm leaning toward a heater problem.