The aquarium with water in it would weigh about 1750 pounds. If you figure extra water, a person, rock equipment, base et al you could figure about 3000 pounds. Take the footprint of the aquarium, and you get about 250 pounds per square foot. Typical houses are built for a 45 pounds per square foot live load. Depending on if the joists were running perpendicular or parallel to the aquarium it might be ok. Not knowing the location or joist alignment though still makes it close. Think about is as per a large book case or a water bed.