I have a 16-Angler and am able to carry 27.5 gals - all comprised of 3-tanks under the transom well >> they are:
one 6-gallon tank
one 12-gallon tank
one 9.5 gallon tank.
Looking at the stern, from the cabin > I have placed the tanks (starting on the left) 6-gallon, then 12-gallon, then...