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Watch for bowing and empty out as much water as possible. Scoop any livestock into any Tubs an plug a air stone in just I case it cracks. Glue could work but I would never trust it
You can order a new top trim piece. I'd watch the front panel to make sure it's not bowing. If it does you might need to lower the water level so there's not so much pressure. Bottom line is, those tanks are designed to have the center brace and I wouldn't risk it for long like that. Have you ever seen 55 gallons of water spill onto the floor? You really don't want to! It would worry me, but I hate cleaning up gallons of water on my floors. Ask me how I know.
Other things you can do: get a piece of 3/8" glass about 4" wide and glue it to the top of the tank( remove the plastic top edging.)
Option 2: You can silicone a 1"-2" strip of glass, around the entire tank, to the inside of the upper edge of the tank. This will help reduce the ability of the glass to bow. ( I had a tank manufacturer do this to my 75 gal tank and it held great.)
Option3: Just order a new top frame for the tank from the manufacturer.
All these options can manage to save the existing tank and should be less expensive than a new tank ( unless you get a $1.00 / gallon deal. )