The volume of the fish tank is going to be the length multiplied by the width multiplied by the height.
So, V = L x W x H
V = 6 x 2.5 x 2 = 30 m³
So the volume of the tank when full is 30 m³.
So now you want to first find the volume of the tank when it's a quarter full. So multiply the volume by 1/4:
So a quarter of the volume is = 30/4 = 7.5 m³
But you don't want one quarter, you want three, so you multiply this answer by 3:
So three quarters of the volume is = 7.5 x 3 = 22.5 m³
So the fish tank can hold 22.5 m³ of water when 3/4 full.