As given by the question
There are given that the fish tank can hold 15 ft water.
Now,
According to the question, he wants a tank that is 1.5 times bigger.
Then,
Suppose the size of the first tank is x,
That means, in the x size of the tank, there are a total of 15 ft of water
Then,
After some time, he increased the size of the tank 1.5 times bigger
That means, the size of the new tank was 1.5x
Then,
Hence, a total of 22.5 ft of water can will it hold in the new tank.