The only thing I could think of would be to build some sort of base to go under the tank stand. The base would be wider than the stand to span the load of the tank across more floor joists. Think of making a base out of 2x4s with plywood on top, that is significantly wider than the width of your stand.
Or you could figure out which way the floor joists are running, and move the tank to a spot that is better suited to support the sustained load of the tank.
I suppose if your stand happens to be sitting on top of only one or two floor joists in just the right way, it could cause some sagging. It's probably around 300lb, which ya wouldn't think would cause any sagging :hmm4: How does he know it is sagging? Is the ceiling cracking (sheetrock?)?
Just throwing out ideas there... that would of course mean you'd have to take down the tank, or enough to move it and lift it..