I have wasted money getting a structural engineer to give me an expert opinion on whether a 300g with stand and water (approx 2800lbs) would fit on a second floor. After an hour or two of looking and inspecting the framing members, construction etc, the response that I got was "it should" be OK.
Reputable engineers will not put there neck on the line and tell you "yes" because if something happens, then it's there tail on the line since they provided you the expert opinion to move forward.
The engineer I hired could not give a definitive response due to the many variables in home construction. Have you looked at lumber at Home Depot? Not all 2x4's are manufactured the same. Some are from different wood, some have defects etc. Same then with nails.
Also, the engineer would need to determine the approximate weight that is already bearing on the area that you intend to put the tank. Take into account the wall structure, roof structure etc. that is bearing down on that area.
These are just some variables and I get the idea that no one will ever tell you for sure whether it's OK to or not.
So to be on the safe side, brace the supports, which generally means tearing out walls etc if you are on a second floor, or if your lucky and on the 1st floor, you can go into the basement and put up more bracing.
Eitherway, the whole journey has taught me a lesson. It's not worth the money and work involved to to determne for sure whether the tank will be OK.
The best solution the engineer provided me was to put it in the basement or on a slab.
Thanks for wasting my time and money Mr. Engineer.