Unless the 12" original tank is leaking you should reuse it because it sounds as if it worked well until the valve starting leaking. And I've never heard of a 9" x 41" tank, 9x48" is a 1 cuft. You won't get 2 bags of most anything plus gravel in that size tank, let alone have the proper freeboard (about 15").
The unit with the 12" tank originally came for another site, It would foul and need cleaning every five/six months. From the other discussions I suspect the flow was not enough to completely clean the bed. does a smaller tank work better with less flow?