Concolor Cory Not Eating

My friends concolor cory has not been eating and has been lethargic for 1 week now. His tank is a 36 gallon tank. I helped him perform a water change earlier and the fish is not doing any better. His water parameters are 0 ammonia 0 nitrite 1 nitrate. I have no clue to why he is not doing so well and would like some help.

 

Allow me to ask a few questions:
How many other Cories are in the tank?
Have you seen him move while the lights are out?
What does he feed his Cory Cat?

 

There are 5 more cories in the tank. He has moved a bit, but not much. He swims a few inches then lays down again. He feeds them shrimp pellets and flakes.

 

Hmmm...a couple more questions:
What is the pH of the water?
What is the temperature of the tank usually?
Another thing to consider would be do they have discoloured gills or laboured breathing? Is lethargy the only symptom?
Any new or removed decor?
What other fish are in the tank?
Sorry to leave you with so many, just trying to rule out some ideas.

 

1. Around 7
2. 76-78
3. Might be breathing a bit hard and his gills look fine.
4. Recently added duckweed.
5. 1 Pearl Gourami 1 Honey Gourami 1 Golden Gourami 3 pictus catfish
Its fine really. Just trying to save his fish

 

could you make a pic of the barbells?