Help ripped betta tail

[Badet222]

Hello there im new here and im new in having a betta fish..i just git him- caui for almost 9 days..he is in a 10 gal tank with silk plants, filter and a smooth pots for decoration. After 2 days i got him tank mates 6 neon tetras and shrimp.. just yesterday when i was about to to partial change of water i noticed him to have ripped tails. I dont know the reason was but im so worried...please help... i just get rid of the neons and shrimp last night and have him his tank alone.thank you