The breeder is 18 inches wide.
the regular one is 15.5 inches wide. The first 40 gallon we bought was 36"X15.5". Then we found one for sale on craigs list and it was 36" X 18". That's why we went to get another one when we found it for sale. It's great, we had been looking for months not finding anything larger than a 20 gallon, I already had that one, needed one larger, then we find 4 for sale within a 3 month period.