What killed my fishtank?

Question
I recently witnessed the most horrible degradation of tank condition in my life as an aquarist.  I was wondering if you had ever heard of this condition before.
I have a 5.5 gal planted freshwater tank.  It has been established for a few months.  In it I had a Betta, 2 Corys and a Pleco.  I went away for the weekend and came home to find my albino Cory dead in the tank.  Petco has a refund policy and since the fish was only 6 days old I brought it back to the store.  The clerk tested my water for everything from ammonia, nitrates, pH and all before finding my water healthy and granting me an exchange.  
The next cory I got died the same night.  I brought it back, had my water tested again and received another Cory from the same batch after my water checked out again.  The clerk told me that they may have had a "bad batch" of Corys since a few died in the tank and a few were brought back so I assumed the deaths weren't caused by my tank conditions.
I acclimated the 3rd Cory in the normal fashion, all the while my Betta was swimming by the bag and displaying his normal dominance.  I added the Cory, changed 1 gallon of water in the tank (since the store clerk said my nitrates were a bit high, but not high enough to kill a fish) and slightly lowered the ph from a mid 7 to neutral and added a bit of aquarium salt.
The next morning, about 12 hours after adding the new fish and changing the one gallon of water  my Betta had already developed a huge patch of what appeared to be fin rot in its anal fin.  It was about 20% the size of the fin, only a half day after the Betta was totally healthy.  By the time I got home from work, his fins were almost completely gone and that evening he died.  The rot also progressed into his body near his pectoral fins, about halfway to his dorsal fin on both sides.
In reading up on fin rot and its treatment and causes, I expected to come home and treat the tank.   I was completely shocked to see how aggressively it progressed in 8 hours.
Also, I noticed that my spotted cory stopped swimming on the bottom and stayed in the middle of the water column.  This morning, my spotted Cory (not the new albino I got the day before) was dead as well.    He seemed to be losing a fin and had a huge sore on his side.  As well, my Albino lost both his pectoral fins.  My Pleco seemed unaffected, but yesterday, 8 days after adding the cory my Pleco has now died.  My tank is completely empty except for the Cory that I believe introduced something sinister to my tank.
I am at a loss as to what happened in 24 hours to cause this.  While I understand that lowering the pH could have stressed the fish, I find it hard to believe it could have done so in a manner that would have killed them so quickly, especially from fin rot.  I thought there was a slight chance I forgot to dechlorinate the water, but since fin rot is bacterial and chlorine kills bacteria, I assume this is not the cause.  As well, the fish was not gasping as though it was being poisoned by chlorine.
I am distraught over what happened, and how quickly it happened.  I was wondering if you had any insight to lend.
Thanks,
Greg


Answer
First I am super sorry that happened, never good to get a nice little fish and have it die or have one you had die. Second, my first and strongest assumption is that the store you got the corys at is not somewhere you should ever get fish at. About 4 months ago, I had a beautiful large marble angelfish, two corys, and a rainbow fish in a 30 gallon tank. Well, I wanted more fish so I went to a local chain store and got some little tetras. Everything was fine and I was happy. I had this angelfish for over 7 years, and had the corys for over 5 years, the rainbow for 2 years. Less than a week later, I came home and to my surprise my angelfish was dead. I was shocked and very sad as he was with me for so long. He was very personable and would come to my hand to feed and was by far the best fish I ever had. Soon I discover that the tetras must have had a bacteria which killed the angelfish. I suppose you live and learn but I will never get fish from a chain store again. They are good places to get supplies but NEVER fish. To start the fishkeepers at the store rarely really know what they are doing and dont care. They will tell you anything to make you think they have a good answer. Also, if they even thought they had a bad batch of corys, they should have never sold them and should have put them in quarantine. Ok, now that I have explained my first assumption I will deal with the disease or parasite your fish seemed to have had. Maybe instead of it being fin rot it was just a vicious fungus, all over fungus. If so, that would explain the quick movement and killing. Or the fish had it before and you just didnt notice it or it didnt show signs? It could have been anything. It is hard to say now that it is gone and no behavior can be observed. I would recommend taking the cory back and telling them they should not sell anymore due to your situation, but dont get a replacement from that same store or any chain store. Find a GOOD private store. You might have to travel alittle but trust me its worth it. You should be safe and comepletely clean and re-start our tank. Also, replace the filter media as some left over bacteria can still be in there. It is likely that the cory had a bacteria and somehow could live with it but the other fish just couldnt take it. I am sorry again for the situation. It pains me when I go into a chain store and get supplies and look at the fish. So many of them are diseased or crowded and really abused, not knowingly but from ignorance. Let me know if you need more help either now or on the future. I hope you dont get discouraged by this incident as fish keeping is difficult but by far worth it. I hope I have helped you some, let me know anything more you might need.