What material is the finished floor under the tank? As long as it's not tile, it's ok if the floor flexes a bit. It is also important to note that if the floor flexes too much and the stand also flexes too much the tank could break long before the floor would fail. Make sure the stand is well designed too. Personally, I would keep the jack posts simply because the tank is running parallel to the joists. if you do decide to remove the jack posts, at least add blocking between the joists under the tank and a couple joists beyond the edges of the tank to help with deflection.
Here's a great article written by a structural engineer (since 1976).
Residential Wood Framed Floors and Aquarium Weights
His Conclusion: "Aquariums up to 55 gallons can be placed almost anywhere without much worry at all. Many tanks larger than 55 gallons and no more than 125 gallons will be okay, if they are placed in a good structural location and your floor framing is free from significant defects. For example, a 125 gallon tank, on a wooden stand, placed perpendicular to the joists up against a bearing wall, will often be okay without any additional structural support. If your tank is over 125 gallons, then it is likely that you should consider adding supports under your wood framed floor. Please realize that these are generalities that may or may not apply to your particular situation."