The tank he wants is rectangular prism 20.25 inches long, not10.5 inches wide, and 12.5 inches tall. how much water will the fish tank hold?

The tank is in the shape of a rectangular prism. The volume of the rectangular prism is the product of its length, width and height.

Volume of rectangular prism = Length x Width x Height

Length of tank = 20.25 inches
Width of tank = 10.5 inches
Height of tank = 12.5 inches

So, 
Volume of tank = 20.25 x 10.5 x 12.5 = 2657.8125 cubic inches

In order to find how much water the tank can hold, we have to convert cubic inches to liters.

1 cubic inch = 0.01639 liters approximately
So,
2657.8125 cubic inches = 0.01639 x 2657.8125 = 43.5 liters 

Thus, the tank can approximately hold 43.5 liters of water.