More than likely whoever made this sump had the return pump in the larger end area and the filtration- socks or something- in the smaller section. Just guessing here. You might really be better off using a 40 gallon breeder as a sump, or asking Tony to make you a sump to fit your cabinet as that would be so much easier in the long run. I too returned to the hobby after decades and spent a small fortune making lots and lots of small mistakes by not going with what I needed to start with .