We alternate between salmon oil and organic olive oil. We use around (estimated) 1 tsp (4mL) for Herschel's 1/3 cup of Canidae and a little bit more for Nala's 1 cup. We also supplement ground flax meal (stored in the dark).
Could you just feed even more? Some dogs just burn through food like that. Herschel, on the other hand, is constantly active and runs on so little. He's like the Toyota Prius of dogs.