Tail biting tail biting tail biting =( its means your fish is STRESSED. uh oh! Have any ideas on what might be stressing them? deal with them. then review everything else. make sure theres no additional stress... that means no making them flare at a mirror or such. Make sure you turn the tank lights on/off at regular times every day.
Dont add any live things to your tank right now as they could hold infections or small bacteria that wouldnt harm the fish that orginially had it.. but could harm your betta since the stress easily harms their immune system. Plus, the tail biting isnt just tears.. think about it. Its like cuts. When anyone has cuts or scrapes or anything in their skin, it quadruples the chance of getting infected. My fish has the same problem of tail biting. I made sure I took care of everything stress- related, and he's recovering quickly and not biting much more. Make sure everything is extra- clean before adding it to the tank. Feed balanced diet.. preferably varied some with pellets or freeze dried blood worms. You could consider meds to help the tail heal and to take a further step against preventions, but I dont think its neccesary. Just an extra precaution ;) Be careful- good luck!
P.S)Wow... i sound like a know-it-all..... i don't mean to come off that way. Sorry! I just have some experience with it because of my fish George.. and I want to make sure your fish are o.k. You know I spent 30 dollars on a new tank and filter when George started tail-biting because I was so freaked out? I didn't know anything then. :[
P.S.S) Your fish are freaking gorgeous! :D