If you don't have a DSB in your main tank, I think it's a good idea to go for one in the fuge. I read that the DSB in the fuge should be 10% of the area of the substrate in the main tank for good denitration. The DSB also gives pods a place to live, so I think it would be a good idea. If you do go with a DSB, you should put some eggcrate or something at the bottom to stop the lowest portions from getting disturbed.