Your math looks right to me - ie a little over 100litres maybe... The beauty of metric is that it's so dead easy... I'm assuming that was the overall dimensions of the tank, so allowing for some headroom you get about 100litres (108 if filled to the brim maybe). There's about 3.785 litres per US gallon. If you approximate at 4 you get the 25gallon, but it's actually a little more than that... like maybe a bit over 26 gallons...