Oswald, my betta fish

Question
When I bought my betta fish, he was the one who seemed to need the most care (he did not have a very nice color and his fins were a bit ragged), and I felt bad for the little fella, so I decided to buy him and hopefully make him better again. For a couple months he became really active and spread his fins out wide, and I really felt like I had helped him a lot.
When I left over the summer, I left Oswald under my neighbor's care. When I came back I realized that Oswald's top fin (he is a Half-moon betta) is very droopy and falls to one side of his body. It looks like he can't move it because when he swims he tilts to the side on which the fin is dropping, as if it were heavy. Also, I am scared that he might have fin rot or something because I looked carefully at all of his fins and they seem to be more ragged and a bit shrunken.  There are tiny black spots on them and it seems that the contour of his fins are lined with black in some areas and whitish or brown in others. The fins do not make a "halfmoon" as they do in pictures I have of him from last year.
I am not an expert, but since I love animals, I really care about my little Oswald. If he does have fin rot, what can I do? I live in Manhattan and the only pet-store I know nearby is Petco. I am not sure that they are the best place to get any kind of medicine (except for water conditioning and this thing called "bettafix" that I use in an attempt to fix his damaged fins). What are some good websites or any other kind of store I can go to to get him what he needs, considering that I am not an expert?

Answer
Hi Juliette,
Change all of his water using a good water conditioner such as "Stress Coat."  Add a little salt to his water.  If he is in a 5 gallon tank, add one teaspoon of diluted aquarium salt to his water.  Bettas should be in heated 5 gallon tanks.  They are a tropical fish, and need 80-82 degree water.  Under this temperature, they are cold, and will get sick.  When raising the temperature of the water, so this very very slowly.  Raise it one degree every two days, as bringing up the temperature too quickly could be deadly.   Vary his diet.  Feed him daphnia, betta pellets, betta flakes, plankton flakes, bloodworms, mealworms, glassworms, and brine shrimp.  The more you vary the healthier he will be.  Feed him in small amounts 2-3 times daily.  Example: 2 pellets for one meal, or 2 flakes for one meal.  His tummy is the size of his eye, so we must be careful when feeding him, not to overfeed him.   He should have one cooked frozen pea a week with the outer layer removed.  Cut the pea into tiny pieces, and feed them to your betta one at a time, making sure he eats it, and that they don't fall to the bottom of the tank.  This is so he can clean out his system.  On the day you feed him the pea, feed him nothing else for the rest of the day.  Bettas are prone to constipation, and constipation leads to Swim Bladder Disease.  It is better to avoid this disease by feeding him one pea a week.
For now you must cure him.  He needs a good antibiotic.   Buy "Tetracycline" and follow the directions on the package.  Be careful not to overdose.  Your pet store should have this medication, if not, then you will have to order it online, or see a Vet.  I have never had to deal with websites who sell pills for fish, hopefully, your pet store will have this medication, or something similar.  You must act quickly as once a betta is infected with fin, and tail rot, this disease quickly spreads, and eats away at a fish causing death.
Bless your heart for trying to save this little fish, and I hope he will recover soon.
Lynda