I have an old (35+ years) 110 long (72x18x18, 0.5" glass) that was built without cross braces. It would bow about 3/8", so I siliconed in a glass center brace for peace of mind. I'm sure it wasn't necessary, as the tank had been used without a center brace for many years prior to me getting it.
125g has no cross-brace... problem?
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I picked up a 135g a little while ago, with no center brace. It is 1/2" thick glass too. It didn't look like it needed a brace, but the top trim needed replacing anyways because one of the corners was split, so I ordered a new top trim from the LFS which came with a center brace. When I took off the old top trim, there was a slight notch in the center where there must have been a piece of glass about 4" wide siliconed on. You couldnt see the spot because of a lip on the trim. I'd definitely try filling it if you're sure there hasn't ever been a top brace. One of my 60g has no brace, but its 1/2" thick glass. Its been up and running for years here with no problems. Either way, fill it up outside and let it sit awhile and check to see how much it bows. Good luck!
Yeah I was told this tank was holding water for some time before I got it... but I will make sure to fill it up in the garage for a few days before bringing it inside. 
Thanks for the advice!
Thanks for the advice!I have a 125 that had it's brace broken by the previous owner. He had a guy make an aluminum brace that just lays across the top. It sounds kinda ghetto but it works and it looks clean. What people are saying is probably right and it doesn't need a brace, but if you're worried you can always get a brace for it like mine has.
