I got helped for free by an engineer type guy at Aspen lumber off highway 52 and i25. Probably helped too that i was buying the lumber to shore up there.
Basically i had pictures of the type of joists and room under tank location as well as a diagram with a lot of numbers. Total span, span between major crossbeams under joists. Height of joist and dimensions of the toes. Length on center between joists. How many tank will fit over.
Mine was cut and dry since i could tell from underneath exact location and it was pretty short span between foundation and steel beam and perpindicular allowing most joists under it. Basically it was pretty easy to tell i was fine as is and i still did a 4x8 cross beam on 2 posts to be sure as it's pretty cheap.
If it was closer to border line i think he would not give a firm answer without being formally contracted and quite expensive. But he is an engineer that calculates loads on trusses, roofs, floors, etc.