My Betta... honestly looked like it exploded?

Question
Hi there,
I've been looking for information since I found Charles dead, which was
about..ten minutes ago. And honestly I don't think there is ANYTHING online
that can tell me what happened.
Charles was about a year old. Two weeks ago he got a little bit of a poofy
face, but I had been being neglectful on cleaning his water so I cleaned out
his tank and got him some medicine and that healed up just fine.
Today when I came home, it looked like his lungs exploded. But only on one
side. There was a giant red blotch (giant being a relative term, seeing as
betta's are pretty small) and his tail fin was shredded.
This is really odd. You see, the only thing in the tank with Charles was a
snail. They don't have mouths. And I always made sure that there really
wasn't anything sharp in the tank that could hurt him.
There is a water heater in the tank, but I touched it to make sure it wasn't hot
enough to burn him.
I just don't understand what happened. He was fine this morning, and I left
for a little over 14 hours and he's dead and, honestly, looked like he was
falling apart. Kind of like something attacked him and tried to eat him. But
my bedroom is locked and nothing could have gotten in there.
Do you have any ideas? I'd show you pictures, but I couldn't bear to take any. I
loved that little fish. Basically he was lying up at the top, his gills were all
poofed out, and from below the bottom of his head till about the end of the
stomach on one side he was red and the skin and flesh seemed to be torn up
and falling apart, as well as his tail fin (his other fins were still intact).
I don't think it was red on the other side, but I couldn't stand to look at him
that long.
I would appreciate any information. Knowing what happened to my poor
Charles will keep me from making the same mistake on another fish, if it was
my fault.

Thank you for everything,
Megan

Answer
Hi Megan,
I'm sorry you lost your little pet.  Your betta had fin and tail rot disease. This disease if left untreated will spread to the whole body.  When disease hits the betta, it is fast, you must get medication right away or it will die.  Unfortunately, you were not home to see him get sick, and so could not help him.  He was probably lying there dead for some time.  Fin, and tail rot if left untreated looks like you described..Fungus, bacteria were eating away at him.
Things that lead to fungus disease are poor diet, bad water conditions, and cool water.  The betta must be fed good food, and have variety such as pellets, betta flakes, and dried blood worms.  Check the date on your pet's food to see if it is not outdated. You must change 25% of the water at least once a week, and more if his living space is small.   Betta's live in 78-80 degree water, when they are in cooler water, they gets sick.  A well kept betta can live 5-7 years.  They are intelligent little beings, and make good pets.
I hope this helps you understand what happened to your friend.
Lynda