If it's just the silicone that gave way, you just need to dismantle that section of glass. Removing ALL the old silicone from any seams then reassemble. Now as far as repairing the support, You may be able to use a piece of acrylic. Bend it so it fits snugly over the edge of the old brace and use some weld-on to attach it to the old brace. It will be time consuming, but will work. Of course you could attempt to just remove the old silicone from the seam that split. Then reapply sili and press the seam back together. This would work if you could get all the silicone out, clean the edges well, and get a good amount of silicone back in between the two pieces. You will have to spread the seam out though in order to accomplish this. If there is a band that runs across the tanks top, you'll need to remove that all together regardless. Same goes if there's one at the bottom, if you remove the whole panel. Your going to need to repair the brace regardless. I have seen some build their own bracing from acrylic. Easy enough tio do if you have the right tools. Removing the entire panel and reassembling would be the best choice.
Also, if the brace just broke on its own, I would strongly advise to reinforce this better.
