In this case, you would be finding the area. Since you're trying to figure out how much water can the tank hold in the inside.
Since this is a rectangular prism, the equation you would use for this is:
l × w × h
For your situation:
Length: 5
Width: 3.5
Height: 2.5
So you would plug those into the equation:
Plug in 5 to L, plug in 3.5 to W, and plug in 2.5 to H
5 × 3.5 × 2.5 = 43.75
When you multiply the numbers you would get 43.75.
The amount of water the aquarium could hold is 43.75 cubic feet.