Okay... been there, done that! This is my first post here, but probably not my last (although it just might be my longest).
About 18 years ago, I bought a used 75 gallon tank that had a leaky corner.
Chances are, you wont be able to get a good seal in just one localized area. There will be contamination on the old silicone and water will wick through this void. You will have to remove the end piece and then re-attach it. To do this, you will need to use single-edged razor blades (Home Depot in the paint department), alcohol (isopropyl will work for the tank; any other for yourself is optional), masking tape (optional), duct tape (more on that later), silicone sealer (just make sure it is 100% silicone, with no fungicides, etc.) and a fair amount of patience. Oh, and since you will be working with razor blades and glass, gloves may be in order, too.
The first thing to do is to remove the top rim. This is usually attached to the glass with silicone. Take a blade (or a sharp knife for this part), and run it between the top frame and glass-- inside and out. After you have done this a couple of times, start to pry the frame off. After a little bit of wiggling (and more slicing and cussing), you should be able to work the frame free. Clean off the old silicone as best as you can.
Now it really gets good. Take a fresh razor blade and cut away as much of the silicone bead as you can on all three edges of the end piece. Then, carefully work the blade into the top of the seam where the front meets the side. Do the same where the back and the side meet. Gently pry the corner apart as you work the blade lower. Eventually, you will be able to remove the end piece.
Now, take another fresh razor blade and scrape all of the old silicone off (I mean ALL of it!). After all traces of the old stuff are gone, wipe the areas with isopropyl alcohol to make sure they are clean. Here's an optional part: Test-fit the side back where it goes and hold it in place with a couple of pieces of tape (masking or duct--your call). Put strips of masking tape about 1/4 to 3/8 inch from the inside corners. This will help you get a straight line of silicone on the joints. If you did this last part, take the end back off. If you didn't... well, you don't need to.
Now take your 100% silicone sealer and run a continuous bead from the top of the front, down across the bottom, and up the back to the top. You gotta be quick here. Now slide the end piece in place starting at the bottom and line everything up. Take a couple of strips of duct tape to hold everything in place. The hard part is now behind you. Now you can run a bead of silicone on the inside corners and smooth it out with a tongue depresser, a popsicle stick, or your finger. Remove the masking tape from the inside before it starts to dry (don't worry too much about little silicone stringy thingies, they are easy to clean up later).
Make sure you put enough duct tape on it to make sure nothing shifts (big woodworking clamps can also be used if you have them). Go away for at least 24 hours. Don't touch it. Don't even look at it. Okay, you can check on it if you must.
After the silicone has cured (at least 24 hours... 48 would be better), remove the duct tape and trim off any stringy thingies. Blob some silicone in the channel of the top frame and stick it back on. Go away for another 24-48 hours.
Now for the moment of truth: set the aquarium outside on a level surface (or its stand) and fill it up.
That's how I did it 18 years ago in what is now my 75 gallon planted freshwater aquarium. Good luck.
Even more stuff about this can be found here:
http://www.wetwebmedia.com/aqrepairfaqs.htm
(And since you're in the Dallas area, PM me if you have any questions)
