how important is the center brace?

That is a basic 125 gal tank. All glass or Perfecto makes them with built in overflows. Most any fish store can get you one. Now if you want acrylic that's different I honestly can't help you except to mention Sealife Systems as a good company. I had a 65g peninsula tank from them that was nice plus they make sump and refugiums.

 

several questions, whats the thickness of the glass on your tank? second it that brace original? and is it made out of glass, lastly how much of a bow was there when the tank was full?

I had a 90 gallon that had the same kind of bracing system, and unknown to me it had come lose on the back also and was that way for quite some time at least a year or so. Did the same as you did, siliconed it back on and refilled the tank. Used that same tank for 5 more years till me and my son dropped it during one of our moves.

To be honest with you I really would not worry about it to much now that you have repaired it. Most of those tanks that have that type of brace have allot thicker glass than the ones with the black plastic top edge with the plastic center brace. Silicone will dry just fine, I have siliconed things on my parents boat underwater and it cures just fine and thats not under water. It also get to full strength within 48 hours.

as far as the silicone turning yellow, no it isn't getting brittle, silicone doesn't do that. It never gets hard, I have a tank or two in my basement right now that are over 20 years old and both of them have the original silicone in them (one is a 45 long and has water in it now) and the silicone is just as soft and pliable as it was when it was put on them. It just gets dirty and I would guess thats whats wrong with yours.