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It should be okay, but just to be safe I would put it so it straddles the joists in your house - so that the weight is distributed over several joists, instead of along one.
I currently have a 55 gallon upstairs with no issues at all, and the weight is on 4 legs - one on each corner of the tank. Spread over several joists, I feel comfortable moving my new 125 gallon up there next week
Take a look at the wall you want to put it on. If its an exterior wall then your ok because (assuming the house was built up to code) there will be a floor joist every 16". If the wall is an inside wall then look to see if its running parallel or horizonal to the long exterior walls. Usually floor joists will runn horizonal to the longest exterior walls of a house. Most houses also have at least 1 center beam that is supported by posts in the basement. Placing a tank along the outside walls or along this center beam will be best as the wight will go directly down on an already existing load bearing wall.
If you place the tank along a non load bearing wall then you should try your best to get the tank to spread its weight over as many joists as possible.
I should've brought that up... I live in a newer townhouse (actually a twin-home), and am putting my 125 gallon along the common wall with my neighbor. It's equivalent to an outside wall so I'm not concerned about weight...