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Ok, now on to the answer.
Water weighs approx 8lbs per gallon. So 125 gal
of water would weigh 1000lbs. Acording to all glass aquarium
website their 125 wights 200+ pounds empty so just the tank full of water would be in excess of 1200lbs.
If you take proper care in placing the tank aganst a load bearing wall and make sure the tank's weight is spread out perpendicular with the floor joists you should have no problems. Placing the tank lenghtwise with a wall should assure you of this if the wall is load bearing. This will spread the weight out over several floor joists. Also if you can get a section of plywood and cut it to the dimentions of the stand and place this on the floor then place the stand on the plywood this will also help spread the weight out. Most stands do not have a solid bottom so the weight is not spread out as much. If you ever had a tank on carpet and then move the carpet you know exactly what I mean because usualy there is just a thin border all the way around the tank where the carpet is mashed down.
Make sure your tank is level before and after filling with water. If the tank is not level that will cause stress at the seams of the tank and could/will cause the seams to fail.
Placing your tank on the second floor is no different than placing a tank on the first floor of a house with a basement.
More info about your house/apartment would be good so we can provide more accurate advice.