I would do everything I could to minimize any "point loads"...orienting large tanks so they span several joists, preferably over a load bearing wall below but i think depending on how old the house is 3500 - 4500 pounds i think would be a safe marker 18 - 30 people jumping up n down on top floor all day wouldn't bring the house crashing down many people set up gyms upstairs with just that 2000 pounds pluss what do you think a waterbed weights in at ??? you think ppl put water beds upstairs a king size water bed can hold 235 gallons depending on manufacturer could weight over 2000 pounds easy