if your really nervous about it you could always have a piece of 1/4" glass cut to the exact inside dimensions of the tank and seal it in their. the width of the piece is your preferance but only 1.5-2" would be needed. This would be a form of a center brace. im not sure how good this is and weather it would hold for a long period of time but assuming that you let it set in 24hours i think it would hold pretty good. If it doesnt have a center brace their is no need for one. generally the glass would have been thicker with this in mind. i would not worry about the tank breaking, but over time the seams may give way due to the pulling of the front and back piece's of glass!! something to consider, and if you do want to add the piece of glass in their, you would only need to empty the tank half way, be very carefull to not get any of the silicone in the water and have some towels and a bar clamp to pull the front and back to the glass piece while the silicone cures. ive done it before and their still holding up!! the glass would be very cheap also!!