Technically the depth of the tank is important because water absorbs light waves more than air, so the deeper the water the less light can penetrate unless you increase the intensity. Which is why generally you find aquatic plants near the banks of rivers and lakes where there is adequate light for photosynthesis. Blue light penetrates water better than any other colour, so it is not surprising how important blue light is to plants; red penetrates much less, so you don't see red leaf plants in deep water in nature, only shallow streams, ponds, swamps, etc., because red leaf plants need more red light to be red.
However, a couple of inches will not be that significant, and as WisFish mentioned most standard tanks are within this range; if you were talking a foot or something, it would be a different story. My 115g and my 90g are both 24 inches (outside measurement), my 70g is 20 inches, my 33g is 18 inches. I go with approximately one watt per gallon of T8 on all of them, and all my plants grow equally well in all of the tanks. There is less than 1 wpg on the 115g and 90g (80 watts total on each) and 33g (25w).
As all the regulars know by now, I am an advocate of minimal light over aquaria; the least amount of light to grow the plants is the maximum. My reason is solely the fish. The forest fish as I call them come from dimly-lit waters and I believe will be less stressed with minimal light. On Saturday I happened to be in the Tropical Freshwater Gallery (including the walk-through Amazon rainforest section) of the Vancouver Aquarium and Marine Science Centre, and thanks to recent posts here on this issue I was noticing more than previously how dimly lit the aquatic displays are. The several hundred gallon display housing a couple hundred cardinal tetras, Sterbai corys, Characidium, Silver Hatchets, and a couple others that escape me, has Echinodorus bleheri growing in light sand, and the plants have several runners each rising to the surface with daughter plants all along them. There is less light penetrating that tank than even mine, yet the swords are flourishing as they do in the Amazon basin. The sand substrate, which looks like plain whitish silica, seems dull due to the low light, not bright blazing white as it looks under aquarium lights. And the fish are relaxed and sparkling. The displays in the gallery are authentic, Lee Newman the Curator of Tropical Waters has taken several collecting trips to Amazonia, and is himself an ACA recognized authority on dwarf cichlids. I regretted not having my camera with me, I will next time I go and post some photos. The SE Asian display is similar, very low light, housing clown loaches, Dennison barbs, hygrophila plants, and so forth.
Last edited by Byron; 02-01-2010 at 04:03 PM..