HCA: Hermit Crab Association

My tank is 29 gallons and I have three pp crabs with about 6 inch deep sand and eco earth substrate. Though it may have too much sand? No matter what I do, my substrate won’t retain moisture. At first I thought my heating mat was too low, so I moved it up above the substrate and used a whole gallon of prime treated water to remoisten all of the substrate (I did this a few months ago). At the time we went on vacation so they were in a separate tank for the petsitter. Anyway, since they were in another tank I was able to painstakingly remix all of the sub with water and add more since it sunk down it was so dry. After I remixed it, I put the crabs back in and they all immediately started digging and molting which told me that I should have done that a lot sooner. Another big reason I did it was because just before, two of my crabs had surface molts (they were successful in their molts but it was still very scary). Sadly, about a month or more later, the substrate was completely dry even after all of my hard work of remixing it! Its been a while later and I have tried misting it, thinking that was doing something but it only moistened the top of it because I noticed when I put my finger in, it doesn’t hold a hole. On top of this problem, I haven’t seen two of my crabs for a few weeks so I assumed they have been molting but I haven’t seen any molting holes on the side of the glass even though I usually do. Obviously, they could just be molting in the center of the tank, but I’m worried their caves could have collapsed from the dryness! I went on vacation for two weeks (I got a pet sitter) so I haven’t noticed their disappearance until recently. I can’t remix the substrate because some crabs are molting. Even if I could remix it, it would just dry up again. I’m not sure what to do because misting isn’t doing anything. It’s strange because the tank humidity is perfectly fine, but the sand feels like the desert. How can I help my substrate retain moisture while my crabs are molting? And how can I ensure that it stays that way?

Are you sure your humidity is fine? What is the reading and have you calibrated your hygrometer?

What do you have for a lid on your tank?

My hygrometer says 88% right now and I havent calibrated it. My lid is glass, though it does have a small gap. How can I effectively calibrate my hygrometer? Now that I think about it, it is suspicious that I haven’t had to mist my tank and it stays humid but the sand is dry… Now I’m worried it’s in accurate.

Here is our care sheet describing how to calibrate a hygrometer:

viewtopic.php?f=26&t=92540

I just calibrated it and it is 4% too high. So it’s not very far off. I don’t understand why my substrate is so dry when my tank is around 80% humidity. Maybe it’s too much sand? But even if the ratio of eco earth to sand was off, I still don’t think it could have such a drastic effect. I still have seen no sign of my other two crabs and the sub still falls through my fingers because of its dryness. Since my lid is glass I almost never have to mist the tank because it keeps in the humid air. Could my infrequent misting be making the sand dry? But why isn't the humid air moistening it? What am I doing wrong? I don’t know how to fix it with two molting crabs. I’m worried for their safety while molting.

If you take something like a chopstick, and push it down along the front of your tank against the glass, all the way down to the bottom....then when you pull it out, does the sand fall into the hole? All the way down, or just at the top? I'm wondering if just the top bit is dry, and the lower part has moisture?

I think you’re right about mostly just the top being dry, because when I pulled the chopstick out the hole only filled up halfway. Which I assume means that the upper half of my substrate is completely dry and the bottom half is moist.

It's normal for the top bit to be dry. You can mist or pour a bit along the edges to try to get the top portion more saturated, but be careful you don't introduce so much water that you cause a flood (you can also use the chop stick method to ensure that no water pools in the bottom of the hole).

Is it safe to pour water onto the sand while I have molting crabs? Should I wait until they are done or is it more dangerous to have dry sub? Also the air humidity is perfect right now but once I add water to the sub the tank will probably be too humid. Maybe in the 90’s for percentage. Is there such thing as “too humid,” or is 90+ okay? I could wedge something in the lid to air it out, but then the substrate might dry out. Also last time I poured a whole gallon onto the sub and it still dried out in a few months. I guess if I keep misting the substrate then it will stay moist, but then again the tank air could be too humid if I do so.

I actually just had a revaluation as to what the problem could be! I have been keeping my hygrometer on the sand in the middle of the tank pushed up against the glass. The crabs always knock it down so it gets covered in sand. Could this be making it read a higher humidity than what the air actually is? Although I’m not sure if this is the case since my sand is so dry. But I am going to test it out by taping my hygrometer to the glass higher up and seeing if it reads lower than it has been. This could be leading to my sand drying out because when I see that the tanks humidity is high, I see no need to mist it. Therefore leading to the substrate drying out. I will update once I test this out- I REALLY hope that this could be the problem. If it’s not then I don’t know why my sub drys out so fast.

Yes it for sure can make it read a bit higher!

So I moved the hygrometer up higher and it still reads normal humidity (in the 80’s). I checked to make sure all of my heating pads are above the substrate (they are) and still no sign of the other two crabs. If I add water to my substrate the tank humidity will be in the 90’s. Is too much humidity bad? Also is it safe to poor water on the substrate while my crabs are molting?

There is no 'too high' when it comes to humidity. Sometimes items are more likely to mold, but the humidity is not harmful. My tank has been in the 90s for months now.

Yes it is safe to pour water along the edges while crabs are underground. Just be cautious that you don't add too much water and cause a flood.

Okay, thank you so much! So far one of my crabs has already come up from his very successful molt. Now that I know there in no such thing as “too much humidity,” after I re-wet the substrate I can continue to mist it to ensure it won’t dry out! Thanks again, and hopefully Mango (my other molting crab) will come up soon!

curlysister wrote: ↑

Tue Aug 20, 2024 11:22 pm

There is no 'too high' when it comes to humidity. Sometimes items are more likely to mold, but the humidity is not harmful. My tank has been in the 90s for months now.

Yes it is safe to pour water along the edges while crabs are underground. Just be cautious that you don't add too much water and cause a flood.

What exactly is "sand castle consistancy"? Is it wet to the touch and shapable, but not dripping when you squeeze it?