Some time ago you had in both Cory Adolfoi and Cory Duplicareus. I bought the duplicareus - which are doing just fine - as the adolfoi had darkened gill covers, making it look like they were bruised, although it was clear that they were not and all of the fish had these dark spots. I've just come across this description of a different cory - Corydoras sp. C121 - which looks like what you actually had. From the description, these are often confused with the adolfoi and I'm wondering if this might be what yours really were. Here's the link:
http://www.planetcatfish.com/common/spe ... es_id=1361
and here's the adolfoi
http://www.planetcatfish.com/common/spe ... ecies_id=1
Interesting....?