Just wondering how much a gallon is in inches cubed. I got a tank from a friend that is 24 in. x 12 in. x 17 in. and the glass is about a centimeter thick. I thought it was 30 gallons at first but now I'm beginning to think it is a 20 gallon. I'm getting a new filter for it so that's why I want to know. I'll most likely get a 30 gallon filter to clean better anyway.