saby 
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[@𝐒𝐞𝐚𝐫](/forums/dis/topics/ask-the-mods-anything?post_id=5685068#post_5685068)
Without expertise in the matter of crustacean-friendly methods of clearing out an arachnoid infestation, I have some thoughts.
Whatever methods you choose to use, if you can keep the crabs in quarantine long enough to confirm the main tank has no surviving mites, it's best to do that.
Sanitising the substrate will be most efficient if you can boil it or bake it to ensure thorough, sufficient heat. I'd be unwilling to try how well the seams and walls on a tank hold up to water that's hot enough to kill the critters, especially in volumes large enough to soak and heat the entire mass of the substrate. Taking it out will also permit you to clean the bottom of the tank and let it dry out completely for extra safety.
In any case, before breaking the quarantine, you should be positive that both the main tank and the crabs themselves are mite-free. Since you've seen them enter the shells, there's no guarantee that some haven't hitched a ride and could re-infest the main tank.
/ETA: after some basic searches, following are advised specifically for hermit crab tank grain mite infestations: remove food scraps and the food dish every time after feeding to minimize mite access to food; change water and rinse the dish daily; find the source of grain mites in your home and treat accordingly by disposing of spoiled goods and heat testing or freezing any containers mites could have gotten into (likely your dry pantry; locate mites by putting some flour on offer as bait; make the flour as smooth as you can; if mites have visited, the surface will be disturbed).
Without expertise in the matter of crustacean-friendly methods of clearing out an arachnoid infestation, I have some thoughts.
Whatever methods you choose to use, if you can keep the crabs in quarantine long enough to confirm the main tank has no surviving mites, it's best to do that.
Sanitising the substrate will be most efficient if you can boil it or bake it to ensure thorough, sufficient heat. I'd be unwilling to try how well the seams and walls on a tank hold up to water that's hot enough to kill the critters, especially in volumes large enough to soak and heat the entire mass of the substrate. Taking it out will also permit you to clean the bottom of the tank and let it dry out completely for extra safety.
In any case, before breaking the quarantine, you should be positive that both the main tank and the crabs themselves are mite-free. Since you've seen them enter the shells, there's no guarantee that some haven't hitched a ride and could re-infest the main tank.
/ETA: after some basic searches, following are advised specifically for hermit crab tank grain mite infestations: remove food scraps and the food dish every time after feeding to minimize mite access to food; change water and rinse the dish daily; find the source of grain mites in your home and treat accordingly by disposing of spoiled goods and heat testing or freezing any containers mites could have gotten into (likely your dry pantry; locate mites by putting some flour on offer as bait; make the flour as smooth as you can; if mites have visited, the surface will be disturbed).