This is a discussion on Petco..... :/ within the Invertebrates forums, part of the Freshwater and Tropical Fish category; -->
it kinda sounds like the fish may of had columnaris.
I don't trust anything from a community tank at Petco. Probably why I only ...
It could have been columnaris. otherwise i believe it could have been... Spirulina? I think thats what it was. I guess either way it was a fungus of some sort, and probably began because they had fin rot. I recently went back to petsmart and bought 1 more guppy and a bristlenose pleco. the new guppy, unlike the others, seems to enjoy exploring and hiding in my large pleco cave. and my bristlenose seems happy and unlike the ones i've previously bought from petco. he seems well fed (he doesn't have the sunken looking belly.)