It all depends on how your floor is laid out: parallel or perpendicular joists relative to the tank, how far from the end of the joists the tank is, etc. I have a 250g tank that runs parallel to the joists that I didn’t add any supports to. But I also have a steel I beam almost directly under my tank and perpendicular to the tank/joists. No measurable deflection.
If you need to reinforce the floor the best structural tool is adding a column in the basement. Next best is to double up the joists under the tank (sistering) or add new joists between the old ones. If you can do it, add a steel I beam instead of extra joists.
Not knowing your specifics, if your tank is aligned perpendicular to the joists and against a load bearing wall you likely will not need any additional floor support.
