Using the Golden Ratio on a tank:
http://gwapa.org/wordpress/2008/01/january-2008-meeting/
Yes, you find a focal point, at about a third point. You try and create a flow - a natural line for the eye to follow - to that point. There are little tricks involved in terms of angling materials, plant selections, etc., to emphasize and do it right, but that is the basic gist of it.
After a while, it is sorta almost second nature. Takes some practice to find it, but once you do, it becomes very natural feeling to find it again.
That scape from above grown in:
http://www.aquaticplantcentral.com/?p=vB53770
http://www.aquaticplantcentral.com/...php?i=5526&original=1&c=member&imageuser=6784
Second link... where does your eye keep going? Keeps going back up to that clump of anubias on the wood on the right, doesn't it?
Look at the wood..... from left to right, sweeps up into that point. From right to left, sharp incline up to same point.
Look up high.... red on left fades out down into green coming down to that point....
Look right underneath it... all those little arrow shaped leaves pointing up to it....
Look at that rock on left side.... arrow pointing to same place...
And remember, it is a natural spot for your eye to go, we do it by like instinct. Used in architecture, in art, all sorts of things.... the golden ratio is created to use how we already "function" on that instinct; a way to just use the way our eyes already work to make it feel aesthetic and "natural" for the way our eyes work.