Why are my fish not eating?
If you’ve added a recognisable source of food for the species concerned then the answer is usually stress.
The first thing to rule out is water quality, most fishes will quickly lose their appetites when exposed to ammonia or nitrite or inappropriate water chemistry.
It’s also fairly normal for anxious fish to prioritise security over foraging, in other words a new fish will often be nervous of entering open water to feed – especially in the case of reef fish or shy, nocturnal tropicals.
Some wild-caught fish have very firm ideas about what’s edible and at one extreme are the species that only feed on live fish or corals. A bit of homework will help you identify dietary specialists such as Butterflyfish and Filefish that only eat living Acropora corals for example.