The one place where I could have a close look at the floor situation under the tank. The reddish rectangle just left of the center of the photo is the face side of a 8x2 floor joist. It sits on top of a triple 10x2 beam.

If you are at all concerned if your floor will hold up the weight of your tank, you'll definitely want to read this article by Kevin Bauman a.k.a. StructureGuy on Cichlid-forum.

Kevin's recommendations are very conservative, so let me give you my 2c as well, which are just from personal experience and gut feeling, not technical expertise. Personally I'd feel pretty comfortable to put most any 4' tank pretty much around anywhere in any house that is not so ramshackle that I wouldn't want to live in it in the first place. The worry starts with 6' and larger tanks, and here you want to be rather safe than sorry, and do you research rather than having to repair a sagging floor. Carefully reading Kevin's recommendations, and actually having a close look at your floor will go a long way, an you might even want to ask a builder to look at your actual situation.

Here is what things looked like for my eight foot, 240G tank with two 45G sumps and a super heavy real rock background.

Diagram showing the floor construction in my living room, and the positioning of the 240G tank on that floor.

As you can see, the tank stands against a strong supporting beam with a metal column right next to it, but only with the short end. The long side of the tank runs parallel to the floor joists, and the tank is only directly supported by two of those, although the floor obviously distributes the weight over more joists. The wall running along the long side of the tank is a non-load bearing wall on the ground floor; that is it adds to the load on the floor instead of supporting it.

I send this drawing and my description to Kevin, and here is what he had to say:

"Your tank location is partially on the beam & partially parallel to the joists. My "gut feel" is that it looks pretty scary. (But then there are lots of guys in the forum that put huge tanks on upper floors and somehow get away with it.) Your biggest concern should be the shear stresses at the end of the 2 x 8's. The thing that you really need to be sure of is how the 2 x 8 joists connect to the triple 2 x 10 joists. If the beam is lower and the 2 x 8's sit on top of the beam and overlap a little that is the best scenario. If they used metal joists hangers, that is a good connection too. If they notched the 2 x 8's or simply nailed them into the beam, then I would be very concerned. The failure mode would be a horizontal crack in the 2 x 8 at mid-depth at the end of the joist.

You might find that your tank isn't quite level when you fill it with water. When the floor joists deflect one end of the tank might slope down toward the middle of the joist span."

With this advice I went in the basement found out that the floor joists are mounted on top of the supporting beam on the left as well as the outside wall on the right. I would have loved to ad some support to the ceiling to be on the safe side, but my basement in this area has a lowered ceiling, and to get to the joists I would have to completely remove this ceiling and re-install it afterwards. This seemed like a huge amount of work, so I decided to fill up the tank carefully, and see if the joists would deflect as all. Today, about 2 years after first filling thank, it still stands 100% level, so the weight seems to be sufficiently supported.

Update March 2013:

The tank is still up and running, perfectly level, and there have never been any problems with the floor.