Dead Cory's

[R.A.C.K]

I can't seem to keep corydoras alive for more than a week. 38 gallon take over two years old. Tank mates include Angelfish, cardinal tetras, one blue ram and a couple ottos. Live plants with DIY c02. Water temp is 78 deg. I haven't checked the ph but all of the other fish are fine. Any ideas what could be causing it? I've gone through about three.