fuzzy hole in bettas fin

Question
I have a 10 gallon tank that my betta (Dragon), pleco (Dynasty)and platyfish (The Spartan) used to share, and a few months ago we bought 10 tetras  (all named george) and 5 zebra danios (Steve 1-5) to add to the family. One danio and 5 tetras didn't make it through the first week (the tetras fell asleep near the filter and got sucked in, and the danio was attacked by the biggest danio, who apparently has a betta complex). but after that the rest had no problems at all. On sunday we decided to add some more tetras so we would have a larger school, and figured that ten more would probably leave us with between 10 and 12 tetras total after the first week. But last night (Monday) I noticed what looked like a hole through Dragon's right fin, with some white fuzz on it, and seeing as it was late, i waited till this morning to see what would happen. The hole is about the same size now, but I think the fuzz may have grown. Other than that, he's swimming and eating fine, and seems to be the same as always. I'm worried that the new tetras may have given him something, or that the Danios have attacked him (he completely ignores them; only plays with the Spartan, they chase each other around, and neither has shown signs of being bitten before). I'm worried that he might need antibiotics, which they had last fall (when the Spartan's two brothers died) and I know they shouldn't be dosed too frequently.

Answer
Hi Jack,
A 10 gallon is too small to house all of these fish.  This is what happened,  your ammonia level went up when you added the tetras causing your Betta to get fungus.  Fungus is a catching disease which is due to poor water conditions.  You must do a water change, check your ammonia, and if it isn't down to zero, you must do water changes with a good water conditioner until you get it down to zero or safe.  When your water chemistry is right, ammonia 0, and nitrates safe, you will then have to medicate.  If left untreated, it will eat your betta, and other fish alive.  There are many cures for fungus at your Pet Store, and they are all good.  Do not wait, time is an important factor to cure your betta.  Do a good water change, check your chemistry, if not right, do another water change until you get it right.  You must never ever add 10 fish to a tank, the water chemistry changes almost instantly, and the fish die, or get sick.  Overstocking a tank is one of the worse things you can do.
I hope Dragon get better, poor little guy.
Lynda