top of page

Humidity - Tips, Tricks & Why It Is So Important!

Updated: 5 days ago

In the wild, Hermit Crabs live in hot and humid environments. Heat and humidity are vitally important in a Hermit Crab tank and can be the difference between life and death.

Why is humidity so important?

Land Hermit Crabs don't have lungs, instead, they have modified gills. Allowing them to breathe on land. To do this, they absorb oxygen through their gills.

To absorb oxygen, they need high levels of humidity. A lack of humidity will essentially dry out their gills, causing permanent damage, eventually leading to suffocation and death.

This is why humidity is such a critical part of owning Hermit Crabs, and one of the main reasons that we never remove our Crabs from their tanks and let them roam around the house or handle them.

Ideal Tanks for humidity

Glass fish tanks, with a glass lid are the perfect home for your hermie. Glass holds heat and humidity well.

Making sure your lid is properly sealed (with the exception of a small gap for bubbler airlines) is the key to keeping your humidity within your tank.

Stay away from lids or tanks with mesh lids/gaps. Examples of these types of tanks include Zoo Med or Reptile One Reptile Tanks. These usually come with mesh lids, and mesh walls. If you already have one, you can always turn it upside down to make it into a topper. Alternatively, you can cut out glass or Perspex and replace the mesh lid, or glue them over the top of the mesh gaps in the walls.

Second-hand glass fish tanks are ultimately the easiest cost friendly option. Checkout Facebook Marketplace, Gumtree or your local community boards for secondhand tanks.

How to raise humidity within your tank

Humidity is caused by heat and water coming together, creating moisture in the air, known as humidity.

Ideal humidity levels for a Hermit Crab tank are between 70-85%. Any lower and your crabs will struggle to breathe properly, any higher and the tank could be in danger of flooding or bacterial bloom.

So how do we achieve what sounds impossible? It's actually quite simple.

By adding heat sources to your tank (heat mats or heat cords) and adding deep pools of water, this will in return create humidity within the tank.

Ideal temperate should be between 27-32.c degrees. Any lower and your humidity will skyrocket, any higher and your humidity will greatly drop.

Deep Pools (Salt & Fresh)

Pools should be deep enough for your largest Crab to fully submerge their entire shell. Having a deep pool is essential for your crab to fully submerge but having wide/long pools is essential for aiding in humidity. If you can, look for pools that are deep and wide.

Sphagnum Moss

Adding dampened Sphagnum Moss to your tank will greatly improve humidity, this can be placed inside a tub or directly on the substrate and should sit up against the heat source. Rehydrate the Moss everytime it dries out. You only need to replace your Moss when it is absolutely filthy and filled with sand. To save on costs, just rehydrate the Moss every few days by soaking it in Primed water.

Dry Moss will soak up any humidity in the air, so it is essential that your Moss is always damp. If your humidity is really low and your moss is drying quickly, fill a small tub with primed water and add the moss into it, place this against the heat source. By filling the tub with water, the moss will remain wet and soak up the excess water until it eventually dries out, this will also make it last longer.


Adding bubblers into your pools will help to raise humidity, as well as keeping your pool water fresher for longer. The bubblers will add more water into the air, which in return creates humidity.

Natural Wood

Natural woods such as Cholla and Driftwood can absorb the humidity within your tank, but, if soaked in Primed Water and placed into the tank wet, they can aid in bringing humidity levels up. If you are finding your humidity is always low and you have plenty of natural wood in the tank, try soaking your wood pieces and putting them back into the tank wet.

Natural Sponges

These are definitely not ideal as they only harbor large amounts of bacteria and take up room in your pools. If you really must have a sponge, only purchase natural sponges, soak them in prime water and place on the substrate (not in the pools) you MUST replace the sponge every 24 hours and STERILISE the sponge every time before placing it back into the tank. This is why we don't recommend sponges; they are a lot of work and don't do much other than spread bacteria within the tank.

Moist Substrate

When preparing your tank, you must ensure that your substrate is damp but not soaking wet. Having moist substrate not only aids in your Hermit Crabs moulting, it also helps to keep the tank humid.

Adding Coir Peat Into Substrate

Adding damp coir peat mixed with your sand (8 parts sand to 1 part coir peat) to your tank helps keep the substrate moist and the humidity up.

Deep Substrate

Not only is deep substrate essential for moulting crabs, but it also means you have less air space. And less air space means you have less air to heat and fill with humidity. Tanks with shallow substrate will have more air space and your heat mats and pools will have a harder time filling that air with heat and humidity.

Sealed Lid

Humidity escapes quickly if your lid is not sealed. Avoid lids with air holes, big gaps or mesh. Only use lids made of aquarium glass or perspex. If you currently have a few gaps and can't get a sealed lid, you can cover the top of the tank with cling wrap to cover the holes. This will help keep the humidity inside.

Raising the humidity in the room

If you can, raise the humidity in the room that your tank is situated. This will help raise the humidity within the tank.

You can do this by adding a Humidifier into the room.

Adding Houseplants to the room

House plants will naturally raise humidity within a room, if you have house plants already, move them into the same room as your crab tank.

Avoid continually opening the tank lid

Whenever you open the tank lid, that precious humidity escapes quickly. Avoid opening the lid when you don't need to.

If you do need to open the lid for feedings or water changes, always replace the lid immediately. Dont leave it off for extended periods of time.

Bring your temperature down

If your tank temperature gets too high, the humidity will dramatically drop. Bring the temperature down to around 28.c and you will notice your humidity will level out.

Hygrometers & Calibrating them

You must have a hygrometer in your tank to keep track of the humidity readings. But what is most important is where you place the hygrometer. Too close to the pools, too high above the substrate, too close to the substrate, too far back, too far forward are all positions that will affect the readings on your hygrometer. The best place to position it is in the middle of the tank, 2-3cm above the substrate level.

When you purchase a new hygrometer, don't be so certain that it is calibrated properly. You should always calibrate new hygrometers upon arrival and old hygrometer's every 6 months.

Calibrating Hygrometers

  • Fill a small bottle cap with plain kitchen salt and a tiny bit of water. Just enough to dampen the salt but not saturate it.

  • Place the salt filled bottle cap and hygrometer inside a sealed ziplock bag and sit out of the tank for atleast 12 hours.

  • Check on the reading after 12 hours, the reading should be 75%. If it is not, your hygrometer is out. Some hygrometers can be recalibrated to read the correct temperature, and some you just need to + or - the difference.

  • Example - the reading is 70% after calibration. That means your hygrometer is 5% off. When you add it to the tank, just add 5% to whatever reading you get. If your tank is reading 80%, its actually 85%. If your hygrometer is reading higher than 75% that means your hygrometer is off and you should minus whatever the difference is.

Things to Avoid

  • Spraying/Misting Spraying and misting is only a temporary fix to a bigger problem, it can also eventually lead to flooding.

  • Misters/Humidifiers Misters and humidifiers should NEVER be added inside a tank, this can lead to flooding and other issues.

  • Soaking the substrate Never soak the substrate with water, this will lead to flooding.

  • Natural Sponges Sponges only harbor bacteria and are a lot of work to actually keep them safe and clean.

What soaks up Humidity?

  • Dry natural wood - soaking your wood in primed water and placing it in the tank can aid in humidity, dry natural woods can soak up the humidity.

  • Dry coir peat - having dry coir peat within your tank can soak up humidity and dry out the tank conditions, coir peat should always be damp.

  • Dry moss - a great alternative if your tank humidity is too high, but if it's too low, dry moss may be the cause. Dampen your moss to aid in humidity levels.

  • Dry substrate - dry substrate will soak up all of your humidity, your substrate should be sandcastle consistency for moulting and for aiding in humidity.

Recent Posts

See All


bottom of page