I live on the second floor of a house where I rent a room, and right now I have a 55g. I found a really great deal on a 125g though. Do you think I will have problems with the weight of the tank?
It really depends. You are going to be better off putting it on a load bearing wall (outside wall)
You also want to spread out the weight across as many joists as possible. Is it a 6 ft tank?
I would get renters insurance. There are plenty of places that will support a 125g, but in my renter I wouldnt dare setup my 135G unless I want it to fall through into my roommates room.
it is 6 ft long. why would it matter if its against a load bearing wall though? isnt all the weight pushing down? and is there a way to find the joists?