Just to let you know It can be done. I have a 180 gallon tank with a 75 gallon sump. I live on the third floor apartment. Tank has been up and running for around 8 months. No signs of stress yet.
The one thing you have going for you, is that apartment buildings unlike family homes have stricter requirements for second floor apartments, and higher. They use different building materials depending on your states requirements at the time the building was built.
Have someone check out the building closely. If you can get a copy of the blueprints that will help. Ask your landlord for all the information you can when you ask their permission for the tank. You will need their permission.
As for a smaller tank being safer, That would depend on what size tank. If you go with a standard 55 or a 75(no sump)there is no problem. But you need to remember even if the tank is smaller, if you are useing a sump that will add alot more weight in a small area. If you look at a 120gallon (the 4 foot tank)with a sump. That would actually be worse than a 180. You would be putting more wieght in a four foot area, versus the 180's six foot area.
It comes down to math, and your support.
Reguardless the tank size, you need to remember that someone is under you. and if the floor fails, someone could be seriously hurt.
I know it sounds like alot. But the blueprints should show if they used 2x12's. or even steel support beams. (That is how they build mine, steel beams, with concrete pads). See if they are 16inchs apart, or 24. Then check to see if you can place the tank so that at least 4 support beams would be under the tank, and at least one good load bearing wall is under the tank. As for wieght, just figure you are going to put a small car there. That is about what it will weight when all is said and done.
