I had at one time for few years a 90g and a 40g side by side, plus couple of 10g and a 20 in other parts of the apt. I was on the second floor and the tanks were just against the walls. I didn't even think of structural risk, because I was just starting out and didn't know much at the time, but no I had no problem at all. The landlord never once inspected the apt in a few years I was there. I'm sure though, if she had seen the tanks I had, she would've made me take down the larger tanks. I'm in apt now, I have one small tank because there's an inspection every few months and contract says no more than 20g. If you're planning on a big tank, I would ask or at least convince your landlord that it's not a risk, or get apt insurance.