any good joiner could reinforce the joists from underneath, my sister has a large hot tub which is outside on decking that is on stilts, i'm not sure how much water is in there but it's a lot more than a large fish tank and it's been fine, i wouldn't worry to much anyway if it's on the ground floor.
Unless the hot tub is constantly full you cannot compare it to a tank. Maximum load bearing capacity & maximum continous load bearing capaicty are two very different things.