General Millipede Care Guide

With millipedes becoming more and more popular in the exotic pet hobby I feel it has become important to make a general care guide that can help newcomers learn proper husbandry to make ensure your animal will not just survive, but actually thrive. 
First and foremost, I'd like to address one of the biggest problems in the hobby. Contrary to what your buddy or some social media influencer has told you - keeping millipedes is very different than how most people keep isopods. What works for millipedes will work perfect for isopods, but not vice versa. Another big problem is outdated and just plain wrong information. The most unsettling part is that books are still published with outdated information.
Now that that's out of the way lets get into basic husbandry. Substrate and diet go hand in hand when it comes to keeping millipedes. In fact, the majority of a millipede's diet is the substrate. One thing I cannot stress enough is to avoid coconut coir (coco fiber, nucifera fiber, ground coconut husk, etc.) or anything that contains any coconut husk.
The reasoning behind this is that millipedes rely on bacteria and fungi that live in their intestinal tract to digest food and the gut biome they have isn't designed to break down the coir. A consequence of ingesting this is impaction, which typically leads to death. There are some keepers who claim that coir is fine in the substrate so long as you provide lots of supplemental foods. That's simply not true - especially for more delicate species. This is an old school method of millipede keeping that we now have seen will work for some extremely hardy (typically small invasive/cosmopolitan) species that rely primarily on surface detritus and - most importantly - they all seem to lay a large number of eggs in each clutch. Being on the smaller side means they hit sexual maturity at a surprisingly fast rate. Despite it seeming like there are species that can be kept with some coir in the substrate - they all experience high mortality rates within the colony. The survivng brood members that are lucky enough to breed will give you a false sense of success before ultimately the entire colony crashes.
 "But millipedes can't digest sand and I see that in my millipede's poop." Coir fibers are long and tend to wrap around things or ball up inside the body. While the sand also cannot be digested, it does not clump together and is easily passed along. Even small vertebrates can become impacted from accidental ingestion. While burrowing, millipedes aren't sifting through the substrate - they just consume. This is where the accidental ingestion comes from. It is simply not worth the risk to use any coir products when housing millipedes. Another thing to avoid is any substrate or substrate recipe that says to use hydrated wood pellets. Unless the pellets have decomposed long enough to allow for the lignan and cellulose structures to break down, it's inedible and is nothing more than a filler. People who sell substrates with hydrated pellets try to claim the wood will break down over time, but that's not entirely the case. In an anaerobic environment such as the substrate, it will not break down in a timely manner at all. In fact, by the time those pellets would even start to break down, the rest of the substrate will be spent and need replacing. The raw saw dust can potentially allow inedible fungi to establish itself which means the substrate would be ruined anyway and needs to be replaced.
Now you're probably wondering what is the best substrate to use for millipedes? Ideally you’d want to use flake soil as your substrate or at least as the base of your substrate. Flake soil is fermented wood that has been allowed to break down long enough that millipedes can consume and extract the nutrients from. You can mix it with plain top soil to stretch it out and fill large tubs, or use the top soil by itself. Whichever route you choose you will need to mix in decaying wood and leaf litter into it as well (even more if you use just top soil by itself). When I say decayed wood and leaf litter I don't just mean dead and dried. I mean partially broken down and ready for consumption. If you can't find anything at that stage then you can make something close to it yourself. Collect dead hardwood and leaf litter and let it sit in a tub of water in a warm spot and let it sit until it feels slimy. You'll also notice an irridescent sheen form on the surface of the water from the natural proteins and oils being released. This could take a few days to a few weeks depending on the initial level of decomposition your starting material is. What this does is break down the cellulose structure and put a biofilm coating of microorganisms that work to break down the leaves. Let the leaves air dry and then feed to your millipedes. If you keep a desert species than you'll also want to mix in additional play sand and clay into the substrate. Something to note is that you always want your substrate to be at least as deep as the millipede is long. So if your millipede is 5" (12cm) then your substrate should be no less than 5"(12cm) deep.
Fruits and veggies can be given every other week for the majority of species and should be seen as treats really. There are obviously exceptions to this especially with certain African species like those in the genus Pelmatojulus who should be given fruit weekly. Yes we know your millipede loves cucumber, just like you love pizza and burgers/fish and chips/whatever junk food is popular in your country. We all know it’s not good for us, but we eat it anyway. Same thing with the millipede. Just because it is something real tasty, it doesn't mean that it's healthy. Protein should be given once a week, or even twice depending on the size of the millipede and species. A big issue of debate is the amount of supplemental calcium they need. People underestimate the amount of minerals that are in decomposing leaves. A good layer of oak and maple leaves covers the majority of calcium, magnesium, nitrogen, and phosphorus. Supplemental weekly protein feedings of: freeze dried minnows, f/t pinkie mice, fish flakes, etc. will cover the rest of the minerals needed to keep your millipede happy and thriving. If you're really concerned about your millipede not receiving enough calcium for some reason, you can sprinkle a little calcium powder in a spot for your animal or put a piece of cuttlebone in the enclosure. The need for a water dish is unnecessary since they get all their moisture needs from misting and the moisture in the substrate.
Something else that seems to be overlooked is the need for heat sources. Some species do fine at room temperature while others will require supplemental heat. It is important you do your research on where your millipede is native to and replicate the temperature and humidity according to what you find. The best way to heat a millipede enclosure is to use a heat pad placed on the side of the enclosure. This will allow your millipede to escape the heat and choose a cooler side if it feels like. Placing it on the bottom doesn't give the millipede much area to escape it and having a lamp overhead will dry out substrate too quick. It will be up to you to determine the size heat pad you need to bring your enclosure to proper tempertaure in your home.
Hopefully this helps new keepers get a quick run through on setting up their new pet for success instead of failure.