Lucy, any time tails are shredding and getting shorter I susepect bacterial problems and fin rot... Here is a great site for help. I did rescues (no more of that nightmare) and between that and columinarus (flexibacter)sp??? which is bacterial in nature I found that salt (aquarium salt) helped much along with medications. Her website will inform you and has photos so you can match up what you think yours has. Cleanest of water as well... Best Sherry

http://www.bettatalk.com/betta_diseases.htm