How long ago has he lost interest in chasing the food and how long ago did he stop eating altogether?
It happens sometimes. It happened with many of my predatory cats and TSNs too. More often than not the reasons are unknown.
It makes me think of new bacterial cultures that your fish is exposed to in the new tank that they have no immunity against: usually the dynamics is like you describe - the fish look fine, start to eat (early stage - contract new bacteria), then lose appetite (bacteria settles and starts growing), then stop eating (bacteria grow). Eventually, their immune system finds the antibodies and kills the bacteria and everything goes back to normal. Other times, antibiotics are needed.
Other reasons could be parasites, harassment, swallowing an inedible object, lazy digestion or even an intestinal clog (catalyzed by high nitrates in a study of goldfish, for example), any changes in the water chemistry (different fish can react differently), diet changes, water temp, etc. And sometimes, as mentioned, nothing makes sense and it just comes and goes. Troubleshoot now. 1-2 weeks are usually fine. 2-3 weeks, I'd start to worry. But as an example, the longest some of my cats refused to eat was 4-5 months. Got thin but survived and rebounded.