How long has the tank been set up? Does your lfs sell chaeto? I started my pod population with some chaeto that was loaded with pods from my lfs. If you ask yours for a bagful for your mandrin, he might sell you some real cheap and then put it in your main tank, hold it down with some rock, or if you have a HOB filter put it in your filter media with some rock rubble so the pods have a breeding area. Get that fuge set up as soon as you can, mean while get a bag of chaeto thats full of pods every few weeks. You have a large enough tank and enpugh LR to keep a mandrin.