Currently, I just let my cory scavenge what the other fish don't eat, but i heard that i should feed them special food, like vegetables. Is this true?

If so, how should I go about doing this?

I have never seen either of my corys eat anything, but this one never really moves. he has been in the same spot for 2 days now. his eyes are fine, and since he's so pale i can see his bones and organs, and they look okay. It's really confusing.

Temp: 76 F
pH: about 6.8-7
I have just a small tank, so i don't have alot of test kits. However, I did test my water at a pet store, and my hardness, nitrite, nitrate, and ammonia levels were just fine. It's a 10 gallon tank, gravel bottom, filter works just fine, with plastic plants, a couple rocks ...