I work in building design and although im not a structural engineer i can size up floor and roof timbers from span tables for standard domestic construction. Back in the day i tried to work out what size tank a floor designed and constructed to minimum standards could support. For UK building regulations it came out as something about the weight and footprint of a filled 75g tank. I looked at US building codes as well, and although they use different units to us (feet instead of meters, pounds instead of kilos) the minimum standards where pretty much the same.
Of course, i dont know any engineer that would design to a minimum standard because they always build in a substantial safety factor to cover their liability. Also if the tank where perpendicular to joist direction and was at the end of the span rather than in the middle, the floor could support much more than minimum design. The floor wouldnt fail in the event of overloading either, it would just deflect more than the permissable amount, at least up to a point.
Given all that if someone came to me professionally and asked this question, i could never recommend anything more than what i knew it was designed for and if in doubt consult a structural engineer.