Ballast is necessary to run the 175w mercury vapor bulbs that are used for underwater lights. Metal Halide ballast will run mercury vapor bulbs. You can find ballast for $30 to $40 on the internet. I ordered from a company in Texas to make sure that I received them by Memorial Day weekend. Their Venture ballasts (BV-V90D6112K)
are $37. The shipping for three ballasts was $10. http://www.1000bulbs.com/175-Watt-Metal-Halide-Ballast/
The Venture ballast is wired for 120, 208, 240 & 277 volts. Cut off and seal the 208, 240, and 277 volt wires. Run the orange 120 volt wire and one of the white COM wires to a GFI outlet. Run the other white COM wire and the wire from the capacitor to the bulb. If you order from somewhere else, make sure that the ballast comes with a capacitor. You do not need an igniter. The Venture ballast came with the capacitor already wired to it.
I soldered the extension cord to the bulb with a cheap butane pen. Solder one wire to the side of the bulb and another wire to the bottom. Cut off the third wire. Lowes carries clear glass bulbs. Home Depot's bulbs are colored white. I sealed the bulbs in the PVC with marine silicone. I originally filled the PVC with silicone, put in the bulb, and let it sit for a day. A week later I was curious if it ever completely cured. I unscrewed the end cap and the silicone was still as wet as it was when I pured it in. I left the two parts open to cure. I'll add a little more silicone and screw them back together.