It sounds like he is defending certain parts of the tank, whilst keeping an eye on the tetra's sizing them up, and trying to work out if they will try and take his territory, is he hiding behind things that he uses often as a bed, a hideyhole etc?He may just be making sure they don't intend to take them away from him, if this is the problem he should calm down in a couple of days!Also he could be ill, or just scared, try and get a good look at him to see if there is anything that doesn't look right!Good Luck
Proud mum of : Gandalf, Lily,(ohm bettas)
Pineapple (male betta)
Jade, Indigo (hm bettas)
a few platys, a few guppies,
a few shrimp,
and whatever else sneaked into my tank while I wasnt looking
If it moves. . .BITE IT!!!