I would tell you if I could find my Takashi Amano book :roll:
Google his name and I am sure you will find it easily.
Basically, it is... Greek philosophers and mathematicians found out long ago: the best ratio that pleases your eye is 1:1.618. To explain. When you drink your coffee, you mix one part of milk with 5 parts of coffee (just as an idea) You have a ratio of 1:5. So when you place your focal point, you divide your tank length into two pieces. One has the ratio of 1.618 and the other the ratio of 1.
How to do that??? Very simple: just measure the length of your tank and divide it through 2.618. Take the result and measure it from one side of your tank. Mark it. The rest is 1.618 (no math there). This is the place for your very special centerpiece, focal point or whatever you call it.