Tank Size

 

Hermit crabs need plenty of space to move around, behave naturally and exercise. I wouldn't recommend a tank smaller than 30 gallons / 113 litres for a few small - medium crabs, sizing up as your crabs grow. 10 gallons per crab will ensure enough space for most sizes of crab, however if they reach particularly large sizes your tank will have to grow significantly to appropriately house them. The size of your tank also affects how easy it is to maintain a good temperature gradient so your crabs can choose how warm they want to be. Tanks that prioritise height over floor space will have to be larger, wide tanks are preferable to tall ones.

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Substrate

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Depth of substrate is very important, six inches minimum, so consider this when choosing your tank's height as sometimes even half the height can be substrate for your crabs to burrow and molt in. You will need to provide ample room for all of your crabs to burrow individually at the same time, if need be. Molting is a regular part of a hermit crab's life and they need space to safely do so away from their tank mates. The most popular mix for substrate is a 5:1 ratio of children's play sand to coconut fibre (coco coir), kept at "sandcastle building" consistency, to help your crab's tunnels stay intact, it should not be very wet or dripping. Regularly misting the enclosure will help keep the substrate damp.

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Temperature

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Keeping one side of your tank at 26-29c and the other at 23-25c will keep your crabs happy in my experience, Watch your crab's activity levels as they will be more active in warmer temperatures, some crabs prefer their environment warmer than others. If they seem sluggish increase your temperatures. Place heat mats on the sides of your tank, outside, and not underneath as you are aiming to heat the air not the substrate. Use digital thermometers inside the tank to watch temperatures and keep an eye to make sure your crabs are safe, a tank too cold or too hot can be deadly. 

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Humidity

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Use a hygrometer to measure the humidity of your tank, maintaining it between 75-85%. Your tank can only be too humid if it's causing mould, otherwise it cannot fall below 75% for extended time as it will cause irreparable damage to your hermit crab's modified gills. They need this humidity level to be able to breath and will suffocate without it. Keeping a spray bottle beside your tank with dechlorinated water to spray your tank a couple times a day usually suffices.

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Pools

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Both a salt water and fresh water pool are necessary for in your tank at all times. Treat ALL water that enters you tank with a dechlorinator like Prime, Tetra AquaSafe, etc. This includes misting spray water and any water used to hydrate their substrate at the start of your tank set up. The tanks should be deep enough for the crabs to fully submerge should they choose to, but with easy access in and out so your crabs do not drown. You can put empty shells at the bottom of your pools so they can move them around and get as much depth as they want. Keep one pool of fresh, dechlorinated water and another pool with fresh, dechlorinated water with marine salt added. Brands like Instant Ocean offer the right salt needed for your crabs health, table salt will not do! Simply follow manufacturers instructions for all products. I have found dechlorinators offer guidelines for entire tanks worth of water, so when measuring for a smaller volume it can be difficult, I always add a few drops for a full bottle of water, never had any issues. 

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Enrichment

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Hermit crabs are natural climbers, often scaling rocks, branches, and other structures in their environment. To cater to this behavior, it's important to include climbable features in their habitat, such as driftwood, climbing vines, and rocks. Providing these elements not only enriches their environment but also helps keep them active and engaged, promoting overall health and well-being. Adding multiple hiding spots like coconut shells is also important for your crabs to retreat to and destress.