I have an old shelving unit that was built by someone in the family or a friend of the family years ago that I'm using to hold my 10 gallon Aqueon tank. It's really sturdy, made of pine boards and beams and held together by large nuts and bolts rather than nails. I have no doubt that it can physically support the weight of the tank, BUT...
The top shelf does seem to be bowing down a bit. I don't think the tank caused this, I think it was already like that. It's had a lot of heavy stuff on it over the years. The gap between the tank and shelf in the center is about 1/32 of an inch or less.
Versus either outer edge:
The tank itself is level (all four corners have the same water level). I haven't noticed anything unusual nor have I sprung a leak yet. I've only had the tank for about a day though. Everything seems stable, the gap isn't getting any bigger or anything. Should I be concerned?