The 500 watts is an equivalency rating and is not valid for figuring how much light you need for a tank. You still only get 65 watts from them. They also get quite hot. It's going to be difficult to upgrade the endcaps on a shoplight to waterproof ones due to the way the fixture is constructed. Does your tank have a canopy? If so, you could order some VHO endcaps, get a reflector, and use the ballast from the shoplight to run 2 40w bulbs. Also, this is probably a magnetic ballast which will get pretty hot so you might want to mount the ballast somewhere other than in the canopy. If you're just looking to hang the light over the tank, then I'd probably use it as is. We have been using shoplights on tanks at the store for years and I have only had to recently start replacing endcaps due to corrosion. The endcaps are about $3.00 for a pair. Give us some more details about how you would like to light the tank and maybe we can offer some other ideas on the best way to do it.
Logan J