I agree. Having a 3 ppm Nitrite concentration, and assuming it could be higher with that kind of cheap test... coupled with the fact catfish seem to be fairly sensitive to nitrite compared to other cultured species... well, I would definitely pin that as the cause. Try the water change keith suggested.

Full article free (but old): http://ciresweb.colorado.edu/limnology/ ... Pub079.pdf

and...

The 96-h median lethal concentrations of nitrite to channel catfish (Ictalurus punctatus), tilapia (Tilapia aurea), and largemouth bass (Micropterus salmoides) under similar water quality conditions were 7.1?±?1.9, 16.2?±?2.3, and 140.2?±?8.1?mg NO2-N/L (mean ± SE), respectively
In layman's terms, that's saying they die in FOUR days at 7 ppm nitrite.