The timing of this thread is perfect. I was just looking for millipede substrate recipes earlier. I use coconut coir in my millipede tank, and when I was browsing Arachnoboards, many other people mentioned incorporating it into their substrates. Some people also mentioned putting fresh wood in their millipede tanks because as the time went on it would decay and release more nutrients for the millipedes. I was at Repticon last weekend, and Josh's Frogs was selling millipede substrate that looked like it had a lot of undecayed wood in it. I think Josh's Frogs knows what they are doing, so the addition of undecayed wood may be beneficial. I also know Bugsincyberspace sells a millipede substrate that comes with flake soil, so it is probably good to also have some decayed wood in the substrate. I personally add calcium carbonate powder (sold at pet stores as "calcium sand") to the substrate.