ADF attacking each other?!

[tallon]

Hi, I'm new here and a fairly new frog owner, but have always been fascinated by these great little creatures. I have found this site while trying to find some answers to my questions.
Basically I have a 20 gallon tank, fairly heavily planted with loads of little caves and hiding places. Until today I had 2 African dwarf frogs (not albino/clawed frogs) and everything has been fine. I've owned them for about a month.
Today I went to an aquatics shop to buy a couple more plants and spotted an adf all alone in a completely bare tank (last one left) not even a plant, rock or plastic ornament to hide behind. I felt sorry for him being all alone, (seeing as from what I have been told they are best kept in groups.) so I decided to take him home.
I added him/her (I have no idea) to the tank and it stayed separate from the other two for about an hour or so. I then added a load of blood worms for them, and they all congregated together for the first time. Almost instantly the new member started pouncing and nipping at the other two adf's. It's mainly going for the smaller one of the two.

I'm worried it will kill my other frogs. It is still staying away from my original two, but when their paths cross it always nips at them.
Is this normal? I haven't heard of, or found another post by anyone describing this kind of aggressive behaviour amongst adf's before.
Is the new guy just trying to become "leader of the pack" or do you think it could be because he's scared? Any other ideas?

The new frog is definitely not a clawed frog if that's what your thinking. All four feet are webbed, he is small, brown and has his eyes on the side of his head.

What can I do? I don't want to have to take him back, but if I must, I will

[Jace]

Is everyone roughly the same size? Do you have pictures of them-especially the new one-that we can see to rule out any physical reasons he may be acting abnomally?

I am worried that you did not quarantine your new addition before adding him to your tank. I made the mistake once of not quarantining a new ADF and it ended up sick and infecting two of my other frogs (out of 6) before I caught it. I have never had a problem with aggressiveness before, but each frog is different and it may take several days for them to settle in together. I would definitely give it a couple more days and see if things settle down with the newbie.

[tgampper]

ADFs are very peaceful frogs. They are often victims of bullying by other tankmates, especially fish. However, I have heard that there are aggressive ADF. This behavior occurs frequently during feeding. You will need to isolate the frog for at least 5 days to observe its behavior and check for any diseases. I think once the frog becomes acclimated, there shouldn't be any problems.