Well, I'm not exactly sure of this particular set up. But I can tell you this: You'll want to ground everything that is metal. For example, if the reflector is metal, ground it. If the bracket that holds the light socket is metal, ground it. If both are metal, and bolted together, then they are bonded together electrically, and only one ground wire is needed for both. You can use a crimp-on solderless connector - one with a spade or eye, and bolt it to the metal of the reflector or bracket or whatever. You can even slip it under an existing bolt, like the one that holds the two together.
All grounds will always go together. The main purpose is for safety. Let's suppose a hot (live) wire were chaffed and touching the reflector, and the reflector is not grounded. When YOU touch the reflector, YOU complete the circuit, and YOU are the ground. Death or serious injury could occur. Normally the ground wire has no electric going through it. If the chaffed hot touches a reflector that IS grounded, it should trip the circuit breaker or GFCI recptacle.
Also, be sure ALL aquarium equipment is run through a GFCI receptacle. This also could save your life.
I hope I wasn't too confusing.