What happened to my tank?

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
Hi Greg
Sorry for the bad loss there, I don't have much insight, but can sympathize.  Sounds almost exactly like what I went through a few months ago.  Bought a couple of zebra danios to add to the few I have.  The next day some of my fish started getting sick.  Hit my cories really bad, lost all but 2 of them & a fry I had(really surprised he made it), lost 2 danios, a female betta, a tetra, one or two others can't remember off hand.  Seemed like it mostly affected my older fish-the ones I've had for 5+ years.  I noticed they would lose color in the middle of their bodies, on the cories it looked like a stripe, they'd roll on their sides, and within 12 hours would be dead.  At first I thought it was fish TB, or neon tetra disease, but I believe those take a bit longer to kill them.  Like you said, this hit real quick.  PetSmart was no help at all, tried telling me it was ich....no surprise there lol.  Only way to know for sure would be to take a fish in to a specialist, and have them do a necropsy on it to diagnose what it had.  

I honestly have no idea what it was, I'm thinking more along the lines of some kind of viral infection-especially killing the fish off so quick.  You should be especially careful in case it is viral, and your remaining cory survived it, he could be a "carrier" of it, thus infecting any new fish....

Sad lesson we've both learned, and I knew better, quarantine all new fish for 2-4 weeks before adding them to the tank with others.

I would recommend not buying any fish from that store for awhile.  Pet stores tend to be on one filtration system, so what ever is in one tank is usually in all of them.  I also wouldn't buy any cory cats from any store for awhile, just in case they get them from the same breeders or if there's something going around in your area.  Something similar happened years ago with angel fish in Florida, a virus spread around to a lot of the angel breeders.

I know you didn't ask for this info, but your tank is too small for all the fish you had in it.  Plecos, all varieties get too big for that tank, and cory cats love to be in shoals of at least 3-5 minimum to thrive, and they need a larger bottom area.  The betta is fine for a 5+ gallon.  An oto cat would be good for an algae cleaner, after the tank's been established awhile-they need good water quality, they're pretty sensitive.  Or a snail or two, some ghost shrimp, not much else can really fit in there.  You eventually would've had problems with all those fish in there.

And, like you said the ph adjustment wasn't likely what killed the fish.  But, large adjustments like that can shock/kill the fish.  You're best bet is to just leave the ph alone, as long as it's between 6.0-8.0, most fish can handle it just fine.

At this point, you have a decision to make.  Either buy a fish and see if it survives with the cory(I know sounds cruel).  If it does die, you'll either need to resign yourself to keeping the cory alone by itself, or returning it.  If you end up returning it, I would completely break down the tank and disinfect it.  I usually don't recommend doing that, but not sure what we're dealing with exactly, seems the only action to take.  
Use a bleach/water mixture, clean everything, gravel, inside/outside of the tank, decorations, filter, tubing, throw out the filter pad.  Rinse well, till it doesn't smell like bleach, then be sure to use a dechlorinator when you add water.  Bleach is just a milder form of chlorine, so that should take care of it.  Then, you'll need to recycle the tank.....

Sorry can't offer anymore insight on it, sometimes it's just impossible unless you have a microscope and know how to diagnose what you're looking at-or what to look for.  
Hope that helps, and again sorry you had to go through that.  I can understand how upset you were, I was too!!
Good luck with whatever you decide to do!  Let me know if you have more questions.

Christy