19 1/4"needed.
whats the aplication? they can be split in to two 10"pro 30s tee the line off and order the extra pro 30 elbow.Also it will fit rims up to 1" and you can set either side over a center brace If trying to use on a 36"frame.
I run a pro 60 with a power head pulling of a UGF Quitest tank I have no water splash noise and take care of a 29G and a pair of fish with a two ounce a day shrimp habit.Water change 30% when nitrates hit 30ppm like once a month maybe.In other words they will grow to handle massive bio loads.