Question 22861
The tank is 20" high.
It has 5" of water in it.
Fish add 1 1/2 " to the height of the water.
He leaves 1 1/2 " to spare.

To fill the rest of the height with water it needs to be
  20" - (5 +3 1/2 +1 1/2)" = 20 - 10 = 10"

He need to raise the water 10 more inches.

Cheers,
Stan H>