A 52 with sump and stand on the 3rd floor. I happen to know our floor is foamed concrete, made to be strong and light, a modern apartment. Also, if you want anything to hold up, think pyramid: any corner is stronger than the middle of a wall; anywhere the run is long and flat is weaker. Think triangles, above and underfoot, and the shorter the run of the wall, the less likely to bow, just like putting books on a shelf: too few braces. A loaded 52 weighs about like a full refrigerator or a couple of real hefty friends standing and talking. Your real danger is leaks. I recommend (from experience) getting a water-alarm. It's a small electrode device (cheap) you lay on the carpet behind your tank. If water ever touches it screams bloody murder. This alerts you to sneaky leaks that can ruin your welcome in a building.