if you have a tube steel stand, it is quite common for there to be gaps often in the middle, as tube steel is almost impossible to get exactly straight so I am told.
every steel tube stand I have had in the last 20 years has been this way. as long as the ends are well supported you are fine.
if you put foam on such a stand, it will be more compressed at the ends and less so in the middle, so you will still have small gap between the steel and the foam. so in that case it doesn't help at all.
but if you don't have a steel stand, then this is a different scenario as chances are the stand is warping and will continue to do so which is bad.
more info would be helpful as well as pics.
chances are if the tank itself is really bowing, it could break. the bottoms of tanks are not supposed to bow.
older style braceless tanks had a degree of bowing on the front and back panels which was normal and expected, but newer braced tanks should not bow.