Don't even bother with the pods... you already have them. Your fuge will populate itself. DSB are usually 4-5 inches. Mine was 5". At least do 4... some do 6" heck some fill up a 5 gallon bucket as a DSB. But it isn't just sand... depth provides the low oxygen areas for anaerobic bacteria to live and consume nitrates. So 4 or 5. Even though... my 3" sand bed in my tank bubbled nitrogen all day.