Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Light goes through glass just fine as long as the rays of light are close to perpendicular. The more of an angle the worse it is, some light will bounce off the glass and is lost.
I think your tank will look better, and the plants grow better if you put the lights over the middle of the tank, not the back. Either centered, or a bit forward. Otherwise every time you look at the fish or plants you are looking at the non-lighted side. Fish show up much nicer if they are lit on the same side as you are looking.
Height of the light:
How deep is the substrate? Usually a planted tank has substrate that is taller than the bottom rim. Perhaps 2" or so at the front and can be made into hills and valleys, so can be a lot deeper near the back. A sloping substrate makes the tank look better. I would go with an absolute minimum of 1.5" at the front and 3" at the back, and more is better.
Then, if the light fixture sits on the glass lid, the bulb is still some fraction of an inch above the rim, perhaps and inch or so.
Height of the light is measured from the actual bulb to the top of the substrate.
Using the old 'watts per gallon' idea, larger tanks could get away with fewer watts per gallon, such as about 1.25 to 1.5 wpg of light from T-12 or T-8 bulbs, hopefully about 6,500K, or a mix of 'plant and aquarium' bulbs and 'daylight' bulbs.
I like a blend like that because the specialty bulbs contain more of the proper wavelengths for the plants, but make the fish look odd. The 'daylight' bulb fills in the spectrum better so the fish look good.
What are the other dimensions of the tank? Length, depth from the back to front?