Cories not acting right...

[fishwatcher]

I have 2 cory cats (1 albino, 1 leopard). I know they're supposed to be nocturnal, but they rarely ever do anything during the day (or at night when the tanks lights are on). I'm not sure what they act like at night, obviously. I see them going up and down teh tank walls (in the lfs), but mine never do that. They only scavenge around the bottom a little, too. Just wondering what the problem is. To be active fish, they act like couch potatoes. The stats are: ammonia 0, nitrite 0, nitrates 5, temp 74, pH 6.4.
Any one know what will make them liven up?

 

[newfishies]

cories are very social, and prefer to be in bigger groups of the same species. they are more comfortable and active with more "buddies". although, it looks like your tank might not have room for any more fishies!

 

[fishwatcher]

Yeah, it's at it's limit, but I do more water changes/vacs than what you normally do to keep it clean. Do you think that's the problem? That they need more?
I saw a store that had small ones... not sure if they are just young are actually some kind of dwarf/pygmy cory. I could get one of those though if you think it'd help.