sudden catfish deaths

[replicate]

I have a tank of Russian Metal Snake guppies and am very attentive to their care. Yesterday, I did a 1/2 water change as guppies require that type of care. Overnight, it seems that the three very healthy catfish (corydorus/albino) I had in the tank were dead. The adult guppies, as well as the fry, seem fine. What do you fish afficianados think??? Thanks so much for your assistance.

 

[Tempestuousfury]

Did you have salt in the tank? What are the nitrogen compound levels?

How old is the tank? How big? have you been feeding the cats algae wafers or sinking pellets?