tetra bullying

Hey all, I have a 16gal medium planted tank. I have a couple snails, 6 glowlight tetras and about a week and a half ago I added a half-grown male giant betta (currently about standard betta sized). Everyone was getting along swimmingly until today, when I caught the largest tetra nip my betta’s...

So two nights ago I realized that one of my guppies had died. I didn’t see anything wrong and I figured that it was just of old age or something since he was an old fish. Then another one died and another! I have recently added two new lamp-eye tetras (in addition to my one already in the tank)...