Everybody in the world who used to use gallons will use UK gallons (like us) The US is the only place that uses US gallons. Aquariums are often refered to by the way they were built such as a four footer. A 24x12x12 tank was 2ftx1ftx1ft and held 12 UK gallons, was 2 cubic ft and each inch of water was a gallon (UK). 30 years ago everybody had one, it was made from recycled glass from broken shop windows and the pet shops paid $12.50 for it. If you have an American recipe it will be USgallons.