To calculate it, you need to know the dimensions of the aquarium, i.e., how big its stand is, the span of the floor joists (i.e., the width of the room), and what the room is otherwise used for. 2"x6" is quite a flimsy dimension for floor joists, so it can't be a particularly wide room. Please also mention the house's construction year.
The aquarium will not go through the floor. However, the situation is not ideal either. I would say it is borderline. There is a risk that the deflection of the floor joists could become greater than desired. The basic problem is that 2"x6" is actually too weak a dimension for that span. The floor weight is already at the upper limit. 2"x7" would have been much better. But if you don't have a better alternative location for the aquarium, I think you can proceed as you planned. I assume now is the most justified time.