Hello; This is confusing relative to what I understand about houses with floor joists.
First; All the joists will be parallel to two of the outside walls and at the same time be perpendicular to the other two outside walls. So the issue when placing a large tank is how the long axis of the tank runs in relation to the joists.
Along two of the outside walls the tank axis wil be running parallel to the joists. This means few joists are directly under the tank. This also usually means that the tank weight will be further toward the middle of the joists. Both of these issues will compromise the ability of the floor to carry a heavy static weight.
In the same house two outside walls will have perpendicular joists. A large tank placed along one of these walls will have more joists directly under it and thus spreading the load more. The tank load will also usually be much closer to the ends of the joists where the joists are supported on the wall sill.
My home, for example, has joists on 16 inch centers using dimensional lumber of 2 x10. Newer homes can have 24 inch centers and may have engineered joists. Either floor can be strong depending on a number of factors including blocking. Either floor can also be compromised by a number of things. I have no way of knowing the actual condition of the OP's floor structure as I have not actually had a look see. I cannot make the assumption that a particular floor is strong or weak from general knowledge of codes.
Thwere are also some common misconceptions about floor load capacity. Here is the web address of an article that talks about the issue.
http://badmanstropicalfish.com/articles/article28.html