I found this and I hope it will help you guys:
Have you ever wondered how much water your tank actually holds? You might not think it's important to know the specifics, especially if you're running a lightly stocked tank, but the size difference between what your tank is called (a 10, 15 or 20 G) and the amount of water that your tank can actually hold can be considerable. It can mean the difference between having a tank that's healthy & easy to care for and having a tank where, despite your best efforts, your fish and other pets have to struggle to survive.
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http://badmanstropicalfish.com/articles/article35.html