You can fix it without draining the tank completely, if you are careful. It is important to remove the old silicone completely to ensure good adhsion of the new. A razor blade will scrape the old off. Make sure you remove it all from both the brace and the front glass. The lower the water level the better. You will need to have a clamp arrangement to pull the glass in to the brace. After applying the silicone slowly pull the front in to meet the brace, but don't bring it so close as to squeeze out all the silicone from between the two. Add some extra, below and above if there is room, and smoothe with a finger to ensure a good bond and appearance. It would be much easier if the tank were emptied, as you could better ensure a good cleaning. The glass in the area the silicone is applied to should be cleaned with acetone or alcohol. The silicone will need several days to cure for maximum strength. It is extremely important to ensure that you get all the old silicone off. There is a solvent available to remove it, but I haven't used it. If you decide to repair without emptying the tank, you will have to keep it clamped for 4 or 5 days, to allow the silicone to cure. Then remove the clamp very slowly to ensure that the new bond is holding. . Also, you could clamp or temporarily brace it till the weekend. It may be fine, but I wouldn't risk it. Good luck.