I would add a second container inside the sump and raised for the refugium. Trouble is - is there enough room to contain the overflow when the power goes out? You can't divide a rubbermaid sump because they bow outward when they are full and that will break the seals of the divide. Thats why you will need a second, smaller rubbermaid container for a fuge if you do it that way. The one for the fuge could be elevated and taller than the footprint of the 12.5 sump and still inside the sump.