I actually had a structural engineer come out. His response was that if my floor could not handle a 180 then the building should be condemned. While the floor of an older building MAY settle a bit (my problem- my floor settled a little and for the past 12 years my tank has not been perfectly level) but it SHOULD be able to handle the weight.
Figure 10 LBS per gallon to be on the safe side (factoring in equipment, stand, tank, water, rock, etc.) So a 180 gallon system weighs 1800 LBS. or roughly 150 LBS per square foot.
To be safe, because I am the paranoid type with these types of things- I put 3/4 plywood covered in 1/4 wood (nicer looking stuff) under the tank, the piece was 3x7. I did this to help distribute the weight over a greater area (I am sure that "mathematically" it only distributed PART of the 1800 LBS over the extended base). I also have my tank up against one of the supporting walls of the building.
Considering that the previous owner of my apartment actually CUT through a few of the floor beams, my floor has held up quite well for over 14 years.
Alfred