I have a well established tank (29 gal) with stable parameters pH 6.0, N3 0, N2 0, and Am .25. All the other fish in this tank are doing great. I had a group of Panda Cory's in there for well over a year all's good until abut 6 weeks ago when I found them all dead when I went to feed the tanks. Once I removed all the dead Cory's I treated the tank with General Cure (I followed the treatment procedure). Waited a month all the other fish are still doing great. I even transferred some of the fish into another tank with Cory's in it with no losses. So I get another group of Cory's quarantine them for a couple of weeks then put them in the 29 gallon where all the fish are doing great. I go to feed the tanks the following morning and the Cory's in that tank are all dead. I did notice some bruising on the bodies behind the gills. I don't have this issue in any of my other tanks with basically identical parameters. Any thoughts?