Next step was to install brackets onto the mount location:
Fortunately for me, there was a stud perfectly centered on this wall between the two lamps where I intend to mount the tank. Then, I have a stud 16" to the left, and another 16" to the right that will both be under the tank, perfectly centered, leaving only 8" of overhang on each end.
I attached 3 heavy-duty shelving braces to the studs with (3) 3.5" lag bolts each. I positioned the braces so that the floor of the tank will be 54" off of the house floor, which is a good viewing height for more adults. Kids'll need a stool, but won't be tempted to tamper either.
While weight is a concern in these situations, I'm confident that this support will be more than sufficient. I weigh close to what I estimate the finished product will weight when filled and stocked, and I can do pullups on the tips of each of these braces individually, with absolutely no hint of protest from the wall structure. They're downright rock-solid, and I'm confident that I've exceeded my support needs by at least 3 fold.
Still on my todo list is to tap into the outlet pictured in the lower part of this image, and run some romex up through the wall to a GFCI outlet and box that will be located right beside the brace, directly above the existing outlet. This will keep the reef tank outlet out of sight once the shelf-cabinet is installed, and there will be no wiring visible at all.