Final answer:
To completely fill the fish tank with water, it would take approximately 51.61 gallons.
Step-by-step explanation:
To find out how many gallons of water it would take to completely fill the fish tank, we need to use the conversion from cubic feet to gallons. 1 cubic foot is equal to 7.48 gallons. So, to convert 6.9 cubic feet to gallons, we multiply 6.9 by 7.48.
6.9 ft³ × 7.48 gal/ft³ = 51.612 gal
Therefore, it would take approximately 51.61 gallons of water to completely fill the fish tank.