To find out how many times a pail with a capacity of 51 ml is needed to fill the fish tank, we first need to determine how much more water is needed to fill the tank to its brim.
The capacity of the fish tank is 250 ml, and it currently has 160 ml of water.
The amount of water needed to fill the tank is:
\[ 250 \text{ ml} - 160 \text{ ml} = 90 \text{ ml} \]
Next, we divide the amount of water needed by the capacity of the pail:
\[ \frac{90 \text{ ml}}{51 \text{ ml}} \approx 1.76 \]
Since we cannot have a fraction of a pail, we round up to the next whole number. Therefore, you will need 2 pails of 51 ml to fill the fish tank to its brim.