If it helps, I lived in an older apartment on the 4th floor, it was a walk up.
At one time I had a 77gal, a 46gal, a 29gal, 37 gal and a 10gal all setup in my living room.
Thats almost 200 gals and my building was all wood, not concrete. Did I mention I lived there for over 2years too.
I had insurance too just to be safe.
I had a friend that was a fancy pants Architect and advised me its best to put tanks against low bearing walls. That meant the out shirt walls, no inside walls. For example, the hallway wall that runs with the outside hallway of your unit. Also the wall along by a balcony, etc.
If your building is concrete then your all set. If your tank is 6ft long instead of 4 ft thats even better to since it will stretch to cover more footage and be better supported. Best of luck in you decision!
p.s. Also to if your really wanting to know where your wall beams are there is this gadget at Canadian Tire for about $50 that is a meter that points a red spectrum at the wall and detects where your beams are. I've never tried one myself but bet it be a cool toy.