Every snail house must have a substrate or else they wouldn't have enough humidity or even drown because of water on the bottom of the tank. They also need a substrate because they dig holes to bury their eggs. Any substrate that they can't or wont dig in is unsutaible. The substrate should be between 2-3 inches thick all over. You'll need to change all of the substrate weekly and freeze.
You can also mix different substrates. Probably the best substrate is one constisting of garden soil and peat moss. If you can get garden soil, even if you only add a small patch of it, it's highly benificial to them.
Must be organic garden soil or compost without pesticides, fungicides or chemical fertilizers.
Holds moisture well, is natural and free. They can lay eggs in it. They will sometimes eat soil for calcium and other minerals.
You will need to check it for other bugs, snails and beetles. Cannot be heavy clay or sandy soil. Impossible to get in winter when the ground freezes. It's also heavier and harder to change than other substrates.
Brands to look for: Eco-Earth and bed-a-beast.
It is light and relatively clean to work with. Holds moisture well, is easily prepared and is great for incubation of eggs. Non-abrasive, natural and non-toxic. Reasonably priced and can be used in your garden afterwards.
Has a slightly funny odor and fiberous hairs that they hate to crawl on. You have to put a whole block in their house.
Sold in bales in garden centers.
Doesn't have fiberous hairs in it like coconut fiber substrate so the snails like it better. Holds moister really well. Is sold in large bales in garden centers so you get more for your money than coconut fiber substrates. You can add it to your garden afterwards to help improve the soil. Great for burrowing, egg laying and egg incubation. It looks nice, is light and fairly clean.
Slightly acidic so it may cause white spots on their shell, this doesn't harm them at all though.
Must be free of fertilizers and pesticides.
Same as garden soil. Also you don't have to check for other bugs and snails.
Must be fertilizer and pesticide free. You have to pay for it and it can be somewhat hard to find.
Fish gravel, sand(including calcium sand), wood chips, sawdust, pebbles/rocks, towel, paper, bark, vermiculite, reptile "carpets", leaves, hay, any peat moss other than sphagnum peat moss, any small animal bedding or any pelleted food, yeasterdays news, carefresh, pine shavings, shredded aspen and corn cobs. Anything else that they can't dig in, anything that will dehydrate the snails, is harmful if ingested, has any chemicals, salt or ink is unsuitable for a substrate.
I don't recommend you try to re-use any substrate because it's so much easier and heathier to use fresh substrate.
If you use soil, peat moss or coconut fiber than you can simply add them to your garden afterward. Rememmber to freeze it for several days before though in case you missed an egg. Peat moss is a great soil conditioner because it helps the soil stay loose, holds moisture and nutrients.
The little white bugs most people see are actually springtails. Springtails are minute, wingless insects about 1/16 to 1/8 inch (1 to 2 mm) long. They can be a variety of colors including white, gray, red, orange, yellow, metallic green and lavender. They get their name from the ability to catapult themselves (leap) through the air three to four inches by means of a taillike mechanism (furcula) tucked under the abdomen. When disturbed, this appendage functions as a spring, propelling them into the air away from the danger source. They're actually harmless and eat decaying plant or animal matter and thrive in the damp substrate. Even though they're harmless, you need to get rid of them because several of the conditions they thrive in are detrimental to snails like a waterlogged, old substrate with old, rotting food matter and poor ventilation. If you clean their house as described on the cleaning page you should rarely have a problem with them.
If you see them in your snails' house then you'll need to check your cleaning practices and ventilation. I have found the above cleaning routine to be the best solution for preventing such things. Also make sure your not keeping their substrate to wet and that it isn't waterlogged (so wet it has puddles on top), because that's bad for snails and encourages springtails.
After that, you'll need to completely clean their house. Move the snails to a spare tank or container with a fresh piece of lettuce. Take out all their food stuffs and feed them to the garbage can. Then take out all their decorations and dishes so your left with just the substrate. Thoroughly clean them and throw away anything that's wood and get a new one because the mites will hid on wood or anything with a similar texture. Now you need to take out the substrate and freeze it. You should now be left with a bare tank. Cleaning it at the kitchen sink with the spray hose is easiest. Using warm water, a small amount of vinegar and a toothbrush, thoroughly scrub out their tank. Rinse 3 times and let dry. Fill with new damped substrate and replace their decorations and dishes. Get fresh food for them and give them a bath before returning them to their home.
Try to give them fresh food stuffs daily and take out all the their old food stuffs daily.