These fish do best with a variety of meaty foods. Krill is not a good staple diet for predatory fish. I would suggest some chopped shrimp, scallop, clam, squid and mussell. Frozen plankton is available commercially as well. You can even try some live ghost shrimp. There is a chance the fish has an internal problem that might be affecting the appetite. Watch for stringy, white feces that often indicates an internal bacterial infection.