36x13x12 is about 25 gallons with baffling, equipment displacemeent, and free board you will probably end up with 23-20 gallons of additional water volume.
I am using a 29 gallon long to build a sump refugium. My refugium will be 12-13 gallons in volume with the sump, equipment and freeboard making up the rest of the 17-16 gallons.
The sump/refugium will provide better gas exchange; CO2 & free nitrgen out, and oxygen in. The refugium will be a place to grow macro algae for nitrate export as well as a place for amphipods and cpepods to to propogate for the benefit of my corals and other filter feeders.
Maxx already covered the additional stability that will result from any increase in system volume.
HTH

__________________

Currently tankless.
20 Gallon mini reef with mated pair of Maroon Clowns given to Rougiem!
80 gallon reef given to Rougiem/Wooster HS.