Silicon was coming off from the back of the cross brace. Going to add a new piece of glass on top of the back wall and connected to the cross brace.
A few things on this topic I didn't think of until after I left:
1) With the current clamp in place, are the walls held such that there is no gap where the center brace pulled out, or is there a gap? If there is a gap you'll want to clamp it well enough that the gap closes. Otherwise you're effectively locking it in a stressed state, and while the add on sheet will keep it from getting (much) worse, it won't put it where it should be.
1b) This may require putting a small metal/wood square section against the back before clamping, even better if it only touches away from the center to spread the load, even better if a few clamps are used. Unfortunately my clamps are all in a storage unit in Texas, but just in case I end up with them before you do this you're welcome to borrow them.
2) My aquarium go-to is Momentive RTV103 (assuming you want black to match the tank). It's what I used to do the repairs on my big tank, and I've talked with tank builders who use it. Reasonably available as well. Unfortunately my spare tube (also a couple years old at this point) is also stuck somewhere in a storage unit in another state. Oddly it's one item that Grainger carries (or did) at a reasonable price, unlike pretty much everything else they sell.
3) The problem with that particular silicone, and all of the other 1 part silicone adhesives out there, is that they don't really cure very far from the nearest edge no matter how long you let them sit. Not a big deal in general tank construction, but a real issue if you are putting on a new plate like in this case. You'll want to either make several very thin strips (no more than 1/2") with a slight gap between each one and air paths to the outside and then seal around the edge a week or two later, or use a 2 part cure silicone. Unfortunately my current adhesives supplier relationships are less than quick on response time so I'm not in a position to recommend something for that.
