How deep of sandbed in fuge

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The tank is a 125 gal and the fuge is a 30gal. How deep should my main tank sandbed be and how deep should my refugium sandbed be?

 

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Thanks, I thought I would order a detrivore kit. Any suggestions?

 

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I was told that the detrivore kit has all the creatures to keep a healthy sandbed and to grow food for fish and corals to eat in the main tank.

 

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Thanks for the advise.