Strange behaviour from my Corydoras

Question
Hi Rachel,

Since purchasing three corydoras two days ago for my 120 litre  tropical tank, one of them has been showing unusual signs.

I can guarantee you that the water conditions are perfect: I did a check for pH, Nitrate, Nitrite, and Ammonia, all of whos readings came back perfectly.

The fish in question just seems to sit there: he doesn't respond to food, other fish, or even me. His other two corydoras friends seem to be fine: I've had corydoras previously, and the others are acting the same as I remember corydoras being. Why is this fish different, and not responding to anything I do? I'm worried he might not do anything, and eventually starve himself...

However, I have heard something about Corydoras being nocturnal... am I right?

Thanks very much!

Answer
Yes, corys love the dark as they are sensitive to light. However, this does not mean they will not come out and jump around like the normal behavior during the day. They should also be active during the day and more so at night. I would guess something may be wrong with him. Corys are my personal fav catfish and it is sad to hear. Perhaps he has a disease or parasite? Does he have any signs? If not, perhaps try some more enticing foods like brine shrimp, thin lettuce, algae crisps, or a different food. Maybe he is just picky and desires a more refined food. Also, make sure no one is picking on him as this could be a reason as well. Finally, he could simply be old. Is he a large cory, around an inch? If so, he could just be old and on his way out. Just make sure and keep a watch on him and his friends to make sure no one picks on them or him and make sure he doesnt have any signs of disease or parasites. Let me know if you need more help.