Originally Posted by Dmitry
It would most likely be ok. I know people with 120s in their apartments in NYC. But if you can get friendly with your super and ask (right after you give him a tip for doing something)...you'd get extra reassurance. I think your super will find out about the tank sooner or later anyway. He'll be in your apartment eventually and see it. And you might be seen bringing the tank in.
Your idea for the move sounds reasonable - that's how most people do it, I think. Save some water a bucket and then add new water for the rest. Keep the fish in a bucket or 2 in the meantime (with a power-head, heater if necessary, etc.) But moves are always risky for the livestock. When I upgraded my tank I lost most of my fish in the process - and I was moving them across the room!
Incidentally, the weight isn't really 125 gallons worth of water plus rock and sand. Rock and sand will displace some water.
Rock and sand is heavier than water, that is why it doesn't float.