Big tank in an apartment???

[Bderick67]

the weight of the substrate, decorations, rocks and whatever else in the tank would technically subtract from the water weight

If it sinks in the water then it is replacing the water weight plus adding the difference. But you could place air ballons in the tank for decoration that would make the total tank weight lighter Look at it this way a 5 gallon bucket of water weighs abot 42 pounds were as a 5 gallon bucket of gravel can be 65-80pounds or more depending on the gravel.

up to 120 gallons you can estimate 10 pounds per gallon for total tank weight and get pretty close. but don't forget your stand either this can add alot of weight too. My stand for my 180gallon weighs 896 pounds

have you seen this thread yet?

http://www.monsterfishkeepers.com/forums/showthread.php?t=74544&page=6

I think you posted the wrong link here. If it is the link you intended then it is way off topic