Im an Architect and Ill do the best to give some rational to the discussion. There have been some good points made so far, but some people are missing the fact that this is a ballon framing system.
Ballon framing is a system in which the walls carry the floor load at levels 2 and above. The floors are "hung" on the wall, similar to how most people build decks. This type of construction is satisfactory as far as distributing uniform loads, however not good for point loads, or isolated conditions like in this instance, an aquarium.
The fact that you have 2" pine floor boards on top of rather small joists does help in the situation as it helps to carry some of the load to adjacent joists even if the aquarium isn't sitting on top of them. Most new construction of 5/8" or 3/4" plywood subflooring wouldn't have this kind of ability.
I do agree that your best bet is to position the aquarium so that it is crossing as many floor joists as possible. Older houses typically use actual size members so a 2x6 is truely 2 inches by 6 inches, however in my eyes, this still seems pretty small. Depending on the span of the room or the length the joists are running will be important in this equation. A shorter span means that the deflection (or how much the floor joists bend) are less, which in turn means the opposing bearing wall of the structure will begin to pick up so of the load, as well as the bearing wall you have the aquarium backed up to. I can't see the span being very long due to the size of your joists.
One concern I do have is the fact that this home may have been constructed to utilize this "upstairs" space as an attic area. Attics conversions typically arn't "beefed" up to the standard capacity as a designed floor would be. If this is the case, I would take great care. For instance, a typical uniform load for an attic is somewhere around 15 pounds per sq. ft. (depending on local building codes) where a second floor design would incorporate around 40 pounds per sq. ft.
If in fact you are living in an attic conversion, there are things you can do, however not much beyond yanking up your floor boards and providing "sistered" 2X6s on the existing to help in picking up the load, and reducing deflection. One other thing you may need to do is to beef up the connection to the wall. If the house is ballon framed, the capacity on the connections may need a little help.
If you have anymore questions, please let me know. As always, with out an inspection of the property and the construction type, as well as lack of calculations, this information is not guaranteed, and I cant be held liable for any subsequent concequences
