Oh peachy, I hope I don't have this to look forward to with my group of 19 sterbai, or there will be a "Cory Fire Sale" right quick!:evil_lol: Do you happen to remember if this is behavior seen in tank raised vs. wild stock? I think people line breed their fish too much. If I'm going to breed fish, I get stock from a few different sources. My cories are part wild, part from one breeder, and part from another breeder to make up the group.However, I had a problem with my previous school of sterbai cories. One male cory was actually pecking out eyes and killing the others until he was the only one left. I researched it couple years ago and noticed others having the same problem with their cories. It is atypical behavior for them but apparently has to do with breeding.
20 cories and 5 discus in a 75g? How is it a lot? This is one's own perception. It isn't overstocked to me. The tank is heavily planted and this alone helps enough to control the water quality. I have never heard of a male corydoras attacking other members of its group. If it is typical, where is the link to prove others have had similar experiences?That seems like a lot of cories for a 75 with 5 discus. I would have to say you are underfeeding so the discus are chasing off the cories. Discus also are bottom feeders so they may defend it if they love what your feeding them. However, I had a problem with my previous school of sterbai cories. One male cory was actually pecking out eyes and killing the others until he was the only one left. I researched it couple years ago and noticed others having the same problem with their cories. It is atypical behavior for them but apparently has to do with breeding.
I would just leave the tank alone and let them sort it out, but you could put the discus in another tank, would be easier than catching cories!