This is a 70 watt HQI metal halide pendant that I made for around $70, in this picture I am just holding it over my 75 gallon:
It would probably have been a little brighter but the bulb was brand new and not fully warmed up, I think they break in for a little while and get brighter.
Here are all the parts:
$29.99 BallastWise.com HID E-B DXE70HID1 70W Electronic Ballast
$20.20 Astralux MH 70W DE 8KK Ultra White Bulb
$10 Brinks 300W Halogen Outdoor Floodlight from Walmart
$6 Grounded Power Cord and two extra wire nuts.
For about $70 plus a little extra for shipping, you can make this light. The floodlight comes with 3 wire nuts, so you only need two more. The wiring is very simple, there is a diagram on the ballast, it's pretty self explanatory. The floodlight comes with 300 watt halogen bulb that you throw away, and a 70 watt HQI bulb fits the same socket.
*The wire that is hanging down from the floodlight is for ground, the setup is not
grounded in that picture, MAKE SURE TO GROUND THE LIGHTING ENCLOSURE/REFLECTOR/SOCKET AND BALLAST.
Here is the light output shortly after turning it on, I am using a different ballast and the setup is grounded in this pic:
You can remove the reflector and socket, and mount them in a hood or enclosure. You could just hang the floodlight, but it would be kind of industrial looking.
I am using this same kind of bulb over a 60cm (18 gallon) tank and it's the perfect amount of light for that tank. I think that one of these along with one or two compact flourescents would be great over a 3 foot tank, or two of these with CF's for a 4 foot tank. I think that a 96 watt CF with two of these hanging 10" above a 4 foot/120cm tank would be perfect.
***Warning: Use this information at your own risk. I take no responsiblity for any damage and/or injuries that might occur.***