ok, here is the whole sequence, beginning with the empty tank.
i used the golden ratio
as a guide for determining depth of water. this consistently produces a pleasing result. to find the dividing line that would separate the underwater and above water portions i first measured between the bottom of the top plastic frame and the top of the bottom plastic frame, finding a distance of 14.4". by multiplying this number by .6, (rounded down from the golden ratio conjugate, .618033...) i determined that the above water area would be 8.64" tall. so, the water should fill the bottom 5.75" of the tank.
here is the tank with a quick and simple hardscape. i have found that it is easy to situate rounded river rocks in a nice arrangement. i also used the golden section for the hardscape, placing the largest rock approximately along this line.
the sand substrate is also an inexpensive, easy to work with material--white pool filter sand. it is important to use light-colored substrates in planted ripariums. the emersed plants can begin to throw a lot of shade as they grow and darken the underwater area. light colors reflect light and brighten the underwater area.
lastly, a layer of light-colored, rounded pebbles tops the sand. these little stones help to blend the larger rocks and sand layer and they also help to obscure fallen roots, leaves and other detritus that would otherwise be conspicuous on the white sand.
...next, live plants!