Final answer:
The value of b in the equation y = ax + b is 6, which represents the initial number of fish Carl has in his aquarium at the beginning of the year.
Step-by-step explanation:
The value of b in the equation y = ax + b represents the initial value of y when x is zero. In the scenario provided, the initial number of fish Carl has at the beginning of the year is the value of b. Therefore, the value of b is equal to 6, since he starts with 6 fish before any months have elapsed and additional fish have been purchased.