That is from the catfish eating algae. They will eat their fill of algae. Their stomach dissolves all the microorganisms and aquatic insects. The algae then passes on out. The meat takes on a rancid dead fish smell. Your choice is either through the fish away or you can put them in a tank at home, keep them alive for a couple of weeks and that problem will fade away. I find this problem real common in winter when food is scarce for the cats.