Thanks to all. I will seek the opinion/advice of a professional.
Maybe the attached sketch can help clear up what I am trying to say, what I am thinking about doing and what the layout be.
The double I-joists on the left in the photo is under the half wall. The aquarium will be beside and parallel to the half wall. The aquarium will also be directly over and parallel to the middle I-joist in the photo. 3/4" plywood panels will go between the I-joists laying flat in the basement ceiling. 4X4 lumber will be laying horizontal underneath the plywood and 4 jack posts will hold the 4X4 up against the ceiling. The proper amount of pressure from the jack posts should support the weight of the aquarium.
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