Your hermit crab needs a good substrate for a lot of reasons

Cypress
Sue Latell answered:
Cypress is a conifer. Coniferous wood/needles are toxic to crabs when ingested. Crabs typically graze on their substrates, making cypress a bad substrate. Cypress mulch according to my herp expert is really best for reptiles that are carnivorous. Cypress has uses only in EXTERNAL application, for most mammals and other animals it has toxic properties, and depending on the species of cypress, the most common is arsenic.

The hermit crabs are able to dig and make tunnels in this substrate the best.

Hi Jan!
I’ve looked up this type of sand as I am not familiar with it. The version I saw was a fine grade. While aquarium sands are ‘safe’ they really aren’t an ideal substrate unless they meet the need of being able to pack and hold like a sand castle. Sand that is too fine will often clump and stick to your hermit crabs. This could be dangerous if it dries and hardens. Sand that is too coarse will not pack properly. The ideal substrate allows for successful molting. The best substrate is play sand mixed with a small amount of cocofiber. There are various suggestions as to ratio but you should have more sand than cocofiber. The cocofiber allows for moisture retention and some aeration. You can add additional items to the substrate too (things like orchid bark, oyster shells crushed, leaf litter). If you still want to use your black sand you might be able to put it to use in your water pools.

Substrates for hermit crabs - The Crabstreet Journal

Feb 10, 2014 - Best Substrate for Hermit Crabs - Based on years of experience from Zoologist Frank Indiviglio Coconut fiber, also known as forest bedding, is a popular choice for a substrate. This material is very finely processed coconut pressed into a hard, dry brick. When you're ready to build your crabitat, moisten the coconut with water so it can be broken apart with your hands. Over time, the coconut fiber stabilizes and allows the crabs to tunnel through it easily without collapse. It's also a good medium for maintaining the proper moisture level in the crabitat. Unfortunately, it's also attractive to fungus gnats, which may also want to take up residence in Hermie's home. Coconut fiber on its own isn't ideal for molting hermit crabs, as it doesn't offer them the best protection against predator crabs when they molt.

hardware store is just as good as hermit crab sand, only cheaper.

The best substrate choice is a combination of sand and coconut fiber or soil. This provides a sand/soil mix that best mimics Hermie's natural environment. If you choose a mixture, prepare your coconut fiber -- or gather your yard soil -- and mix it with sand until it's a diggable consistency. Again, the best gauge is if you can build a sand castle with it. A mixture of sand and soil or fiber will give Hermie a good tunneling substrate that helps maintain the correct level of humidity in his crabitat.

HCA: Hermit Crab Association • View topic - which sand is the best??