Have you checked your nitrAte levels in this tank? Cories are sensative to this also. Are they showing signs of stress? Are there any red patches on them? Are they acting normal?
When you do your water changes are you vaccuuming from the bottom? Keeping cories means although they help you keep your tank clean, that you need to keep the gravel clean for them.
I am not sure what Nutrifin MAX is, but is it for all fish or just cories? Cories will eat any leftover food, but IMO need their own food also. I feed mine brine shrimp pelletts, bloodworms, algea pelletts, tubifex worms and a bunch of other foods also.
Sandy