If you made the red boards so they don't go all the way to the end of the top and instead ended inside the yellow boards, you could make the yellow boards go as wide as you want.
Like this but on both sides:
The ends of the red boards would still be supported by the legs but are now 51" long plus the 1.5" for the thickness of the yellow boards on each end works out to 54" total length. The yellow boards are 30" long as you desire. The tank, being 48" would be sitting directly over the leg because 54" - (3.5"*2) = 47" which isn't much of an overlap but it is enough. The frames would now have four red boards instead of two.
Does that help?