Guppies, diseases

Question
I have a ten gallon tank and it currently has a plecostamus and three female and three male guppies.  For the longest time I was not have any problems, but the last few weeks one of the guppies both females and males will appear as though they have a broken back.  They swim funny but don't appear to have a fungus or ick.  I have treated the tank with anti-bacterial medicine as well as anti-fungal medicine and nothing seems to happen.  Once the fish dies, I remove it and then everything is fine for a couple of days and they I will wake up and find that a new fish has this "broken back" looking problem.  Do you have any idea what is causing this and how I can take care of the problem?

Answer
Hi Kristin,
 This is a disease. It causes permanet damage if you can keep them alive. I think you use nalagram to cure it because it's an bacterial disease. You should ask someone at your local pet store for a medicine for broken back and they should give you a medicine.
   I hope I helped,
     Samantha