In larger tanks glass flexing is normal. However in a small tank like you have if the tank is noticeably (you can tell when looking at it from the side) the glass is aging and starting to weaken (UV light will do this over a period of many years in lower quality or older glass types). Glass gets its strength from its rigidity, as it flexes the strength diminishes. The flexing will also put stress on the silicon glue once aged (silicon losses its flexibility over time with UV light).
In short, replace the tank.
Its always a good time to upgrade to a larger tank when replacing an old one
If you are looking at used, look for a reputable brand and inspect it filled.