Dead corie cats?

[Fishman007]

So about 3 weeks ago i bought 1 agassi corie and 2 panda cories and was planning on get more cories once my lfs had more in stock, so the first week i bought them the 2 pandas died and today i just looked today and his dead body was laying next to a plant but when i got him out of the tank he had a red mark on his gills?
Here are some pics-
















My specs are

0 ammonia
0 nitrites
5 nitrates

Ph-7.6
Temp-78

And my stock-
6 neon tetras 1" each
German blue ram pair 2" each
1 bristle nose pleco 2"
1 banded kuhli loache 2"
Thanks!!!