Albino Disease/parasite?

It's been a long time since I've posted here. I've had my 36 gallon tank for a long time now, excellent filtration (HOT Magnum), perfect temperature, and good readings for the water. Done for me at fish shop, he said he wishes most people's water he tested was a more like mine. This leads me to my current problem. I went to a local fish shop that only sells fish and supplies. I've bought from there before. I bought three new plants from them, and one snail. A couple of days before my girl picked out a guppy she wanted from Petsmart. I just did a partial water change and vacuum as usual and everything was right in the little aquarium world. I even switched the decor around completely. Well, the new guppy was dead. TO fast forward a bit, I've had 4 or 5 fish die in 3 days. Recently, I had a betta in the community. He died, too. Only weird thing is that part of my light-blueish better were PURPLE.....he had grown purple spots...weird purple spots when I found him dead. Now, my cory has this weird white patch growing on it. I also have an emerald cory and it looks like the same thing is happening to him only at his tail end. It also looks like the fin the whitish weird part is attached to is not complete anymore...the webbing is looking "cut". Bad description I know.What is this and what can i DO (IF ANYTHING) TO PREVENT THE REST OF MY CREW FROM DYING?

Looks like columnaris to me. A very deadly disease.

http/www.americanaquariumproducts.com/Columnaris.html

A few things you can do. Fish are like people! Fish can get sick with stuff inside as well as fungus out side. How ever Corydoras have a hard shell called rays. Not scales. So they are less likely to get fungus or Ich. I would put all sick fish in a tank with about 1 table spoon salt per gallon of water. I would raise the temp up to about 78* to 80* as fungus and Ich dont thrive in salt. As to the corys with out seeing them its hard to say. I would be guessing but it could be a fin and tail fungus. You should try to never put new fish in your tank with out having them in a medical tank for about 8 weeks. A 10 gallon tank works well for this. Hope this helps.