Originally Posted by lorax84
Actually those are standard sizes. The tank most people call a 75g (48x18x18) is actually 68.7 gallons, a standard 90g (48x18x24) is actually 88 gallons. I don't know why someone decided to call them a 75 and a 90.
Are you talking American gallons or Imperial ? Maybe that would explain the differences you are referring to.
The imperial gallon
(L)) is 20% larger than the United States liquid gallon (3.785411784 L) according to Wikipedia.