How long and high is the tank?
How thick is the glass?
An aquarium that is 2 foot long x 12 inches wide x 12 inches high, the glass on the sides should be at least 4mm thick, and the base should be at least 5mm thick.
An aquarium that is 18inches high should have 6mm thick glass.
All aquariums without a support strip/ cross member on the top of the tank will bow outwards to some degree. A mm or so is not an issue and is normal. If you are really concerned, you can lower the water level an inch or two and it will reduce the bend in the glass. However, lowering the water level will make it harder for external HOB power filters to take up water, so it becomes a balancing act. The water level in the aquarium should be high enough so the filter can automatically restart after a power failure, but low enough to reduce the curve in the glass.
If you have had the tank for more than a year, it should be fine.