How much weight can joists hold.

[partagas]

I live on the second floor of a house that is around 100 - 115 years old. I would like to put a 120 gallon tank that I have sitting in my garage. But my girlfriend ( Will I am also ) is concerned that the tank will be in the down stairs tenants and they will have a sky light into our place....... I checked the atitic and it looks like they run from center out. Also from the looks of them in the basement they look to be 2x6's. Since they are old, they are true 2x6. The tank would be against a wall that is in the same spot down stairs. So does anybody know if two ( If lucky three) joists next to a wall can hold the 1500 plus pounds? I would love to have the tank up and running, but I have to have proof that the floor can hold it.

Any help would be great.