I did mine between 1.5 and 2 inches. Topped it with a layer of sand. Seems to be doing well for me for the last 3 years or so. Now, I can't compare my experience with others that may be deeper or more shallow.
See the 2 pics. One was is when I first set it up. There's rock and macroalgae all over the place now. I got a couple of emerald green crabs down there. An urchin hitchhiker I snatched from my mian lives there too. I've had a couple of gobies down there in the past, but they just slipped thru the overflow and ended up there. Returned them to the main though.
Had a real "OOPS" once. My main pump went out once and had to wait for an overnite impeller shipment. Hooked up a small ph to keep the fuge with oxygen, but I forgot to disconnect the heater. My heater sensor is in the main and the heater is in the fuge. OOPS!!! I boiled my fuge and everything died.
I recommend everyone buy and extra impeller for your main pump to keep on hand.
Just found the article exerpt too...
Miracle Mud makes an ideal and the most efficient DENITRATOR refugium filter leading to pristine environment for both fish and corals/invertebrates. The fine consistency of Miracle Mud produces distinct aerobic and anaerobic layers that enable it to be both an excellent nitrifier and denitrifier.
- Miracle Mud is the most ECONOMIC refugium filter substrate to use if the goal is to remove nitrate from the system water! Unlike competitors' refugium substrates that will need at least 6 inches (15 cm) of deep sand bed (DSB) to achieve denitrification process, Miracle Mud will only need one inch (2.5 cm) deep. DO YOUR HOMEWORK! Multiply the price of competitors' products by a factor of 4 to 6!