It came up with buddhism hotline, but i under stand. Im returning the tetras today. I think they are nipping the bettas tail or he has fin rot. It happend in 2 days.
Good plan. When the Betta is back to being alone, don't worry about the nipped tail. Clean water and peace and quiet are the best treatment. A good water change. Provided the fin/tail is not bitten to or past the base, it should re-grow in time. But don't let the store talk you into some fungal treatment or similar.