What is the height that fixtures should be off the surface of the water? This is going to be a hung type fixture that I made with 2 x 13W lamps and the MIRO4 reflectors from AH supply. The tank is an open top 10g.
There is no ideal height above the water for a light fixture. It depends on the wattage and type of bulbs/reflectors, and on how willing you are to have light spillage. Assuming there is no glass or acrylic on top of the tank, I think the light bulbs should be about 2" minimum above the water, to avoid any possible moisture problems with the electrical stuff. And, with 26 watts of AH Supply type light, you can't go much higher and still have enough light to grow most plants. A ten gallon tank needs more than the conventional 2+ watts per gallon.
The reason small tanks 10 gallons or less require more watts per gallon is because the watts per gallon rule is not valid. You are seeing the flaw because you are at the extreme end.
Not only that, the watts per gallon rule get thrashed because of efficiency variations among light sources it tries to sum it all up in one easy step.
Plants need a certain amount of light to grow, therefore you need x amount of watts per certain light source. Lets say thats 24 watts to produce that light. You can have a tank thats 6 gallons or 12 gallons and still grow plants. Look how skewed the watts per gallon rule is there.
My two cents. Take it for what its worth...:blah: O