HCA: Hermit Crab Association

I have a smallish crab that has been down I assume for a moult for about a month now and am wondering how long she will take. I need to clean out the tank she is in because the sand is starting to smell (not of rotten fish though). Also I think the temperature is bit hot (30 degrees Celsius) and I may have knocked her because I was moving some sand around and found her shell and accidentally poked it. Would she be alright?


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Hiya Madi.crab!

So crabs don't always start moulting right away after they dig down, they often dig around & tunnel about for a while.

It's hard to guess how much time your crab actually spends moulting, so we try not to disturb them underground... did your crab move at all after you poked it by accident?

And what does the sand smell like? Is it like a beach? Mouldy? Can you describe it?

It may be a good idea to take a pencil or chopstick or straw & insert it next to the glass... you can do this all along the side of the tank to make sure the sand isn't too wet. When sand is too wet, bacteria blooms can happen, causing fiercely bad odours... but if you don't have a bacterial bloom, it may just be natural crab tank smells. (Crabs can be a little stinky... food decays rapidly in 80° heat & high humidity... seafood and veggies alike sometimes become icky... it may not be the sand. That's why you want to use a probe to make deep holes & give the sand a good sniff.)

You probably won't need to change the sand out completely unless you have a toxic bacterial bloom. (Bacteria that grows in wet, dark sand, without air or light is called anaerobic bacteria - it can release toxic gass that smells horid & may endanger your crabs. This is what we're talking about when we say bacterial bloom.)

A crab tank with sand that isn't too wet won't likely need the sand completely removed. Taking out a percentage of sand & replacing it annually or biannually should be enough.

When did you last change your sand?

Also, how deep is your sand & what is it (i.e. is it playsand, cocofibre. ...?)

LadyJinglyJones wrote:Hiya Madi.crab!

So crabs don't always start moulting right away after they dig down, they often dig around & tunnel about for a while.

It's hard to guess how much time your crab actually spends moulting, so we try not to disturb them underground... did your crab move at all after you poked it by accident?

And what does the sand smell like? Is it like a beach? Mouldy? Can you describe it?

It may be a good idea to take a pencil or chopstick or straw & insert it next to the glass... you can do this all along the side of the tank to make sure the sand isn't too wet. When sand is too wet, bacteria blooms can happen, causing fiercely bad odours... but if you don't have a bacterial bloom, it may just be natural crab tank smells. (Crabs can be a little stinky... food decays rapidly in 80° heat & high humidity... seafood and veggies alike sometimes become icky... it may not be the sand. That's why you want to use a probe to make deep holes & give the sand a good sniff.)

You probably won't need to change the sand out completely unless you have a toxic bacterial bloom. (Bacteria that grows in wet, dark sand, without air or light is called anaerobic bacteria - it can release toxic gass that smells horid & may endanger your crabs. This is what we're talking about when we say bacterial bloom.)

A crab tank with sand that isn't too wet won't likely need the sand completely removed. Taking out a percentage of sand & replacing it annually or biannually should be enough.

When did you last change your sand?

Also, how deep is your sand & what is it (i.e. is it playsand, cocofibre. ...?)

Ok Magnus didn't move when I accidentally poked him. I'm a bit worried that I disrupted him. My sand is playsand and my tank smells like this every now and then. It's sort of like mildew because it smells like wet sand. How do I know if it's a bacteria bloom? What do I look out for? I know sometimes my crabs bury their food by accident so it might be some food I haven't found yet. I just checked around the edges of the tank in the sand for moisture but didn't find any. The last time I changed my sand was probably a month and a half ago but I don't want to disrupt my crab if she is molting. Is this good so far?


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If your sand is sand castle consistency (not wet - no moisture appears when you squeeze a handful, and it will hold its shape) you should be okay. When you insert a pencil or whatever, you're looking for water pooling at the bottom, sand too dry, and smell. If you hit a bacterial patch you'll smell it (it's not a subtle odour).

If you changed your sand that recently it probably won't need to be changed for at least another 6 months to a year, unless water spills or over-misting are leading to a wet bottom.

The smell is likely just old food. Sometimes crab tanks get a bit smelly.

How deep is your sand?

LadyJinglyJones wrote:If your sand is sand castle consistency (not wet - no moisture appears when you squeeze a handful, and it will hold its shape) you should be okay. When you insert a pencil or whatever, you're looking for water pooling at the bottom, sand too dry, and smell. If you hit a bacterial patch you'll smell it (it's not a subtle odour).

If you changed your sand that recently it probably won't need to be changed for at least another 6 months to a year, unless water spills or over-misting are leading to a wet bottom.

The smell is likely just old food. Sometimes crab tanks get a bit smelly.

How deep is your sand?

Ok I can make a sandcastle without water pooling, I believe they have just put food around the tank.

My sand is only about 3 inches and I know I should probably put more in but I ran out of sand.


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Without disturbing her, are you able to see her shell or tell if she has moved at all since she got poked?

wodesorel wrote:Without disturbing her, are you able to see her shell or tell if she has moved at all since she got poked?

By the looks of things she hasn't moved at all. The other day before I accedently poked her I could hear her digging around. Should I dig her up or no?


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I don't think you should yet. With three inches she probably can't bury far enough to completely hide, and if she was moving around before then I am not so worried about her having passed. If she got herself to where she could molt then they do stop moving for a week or more while shedding and hardening. Please look into adding more playsand for them, being able to fully bury and dig around to make an underground cave is what keeps them safe.

wodesorel wrote:I don't think you should yet. With three inches she probably can't bury far enough to completely hide, and if she was moving around before then I am not so worried about her having passed. If she got herself to where she could molt then they do stop moving for a week or more while shedding and hardening. Please look into adding more playsand for them, being able to fully bury and dig around to make an underground cave is what keeps them safe.

Ok I will add some more play sand in on top of her so she feels safe and secure. Thank you so much for your help


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Madi.crab wrote:Ok I will add some more play sand in on top of her so she feels safe and secure. Thank you so much for your help

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Only add 1 inch per 24 hours and don't pack it down. This prevents tunnel collapses.

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daws409 wrote:

Madi.crab wrote:Ok I will add some more play sand in on top of her so she feels safe and secure. Thank you so much for your help

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Only add 1 inch per 24 hours and don't pack it down. This prevents tunnel collapses.

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Ok I will do that I hope she survives


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Since she is exposed do not add any on top of her! When they dig themselves under they make a little cave around themselves so they can wiggle out of their old skin and so they don't get packed in tight and suffocate. We cannot replicate those caves safely. Adding sand on top of molting crabs only works when they have that cave around them!

wodesorel wrote:Since she is exposed do not add any on top of her! When they dig themselves under they make a little cave around themselves so they can wiggle out of their old skin and so they don't get packed in tight and suffocate. We cannot replicate those caves safely. Adding sand on top of molting crabs only works when they have that cave around them!

Oh so don't put sand on top?


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Is she your only hermit crab? If she is, wait until she is done and then increase the height so it is ready for the next time she wants to dig. If you have others, it would be better to increase the depth now so they don't have to wait and so this doesn't happen again, just leave the area around her untouched until she is done so that she doesn't get trapped.

wodesorel wrote:Is she your only hermit crab? If she is, wait until she is done and then increase the height so it is ready for the next time she wants to dig. If you have others, it would be better to increase the depth now so they don't have to wait and so this doesn't happen again, just leave the area around her untouched until she is done so that she doesn't get trapped.

I have three. I moved my other two into a different tank so they don't disturb her when I heard her digging(I had a bottle around her before that) so she is in a tank by herself and the other two are in a temporary tank till she finishes(if she does)


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