Final answer:
The tank mates may not be eating the food because they are not used to it.
Step-by-step explanation:
The possible reason why Jacques' tank mates are not eating the food he provides could be that the tank mates are not used to the food.
Animals have specific adaptations to certain types of food and may refuse to eat food that is significantly different from what they are adapted to eat. Just like tigers would not last long on a diet of honey, fish may not be able to eat food that they are not accustomed to.