It depends on the dimensions. For example a 120 long would be better than a 150 high. 180-240 is the ideal size for a Jar. If you're going for silvers or black/blue, you want to go 240-300. Tank's width has to be at least the length of the fish if it's a jardini. Silvers and black arowanas are fine with widths that are less than their length. They are much more flexible than Jardinis. Length of tank needs to be at least 3x that of the fish or more for comfort. 18 inches and above is ideal for height. Height doesn't matter that much since since all arowanas don't get over 6 inches tall.