HCA: Hermit Crab Association

Hey all. I recently upgraded my 20 gal to a 55. In my 20 gal I was pretty good with humidity temps. If anything they were very high towards the end and I was having some issues. Since upgrading to my 55 I’m having the opposite problem where the humidity readings are coming in on the lower end (high 60s to low 70s). Does the size of the tank factor in and should I be doing anything different? I have bubblers in their water pools, a Ultratherm on the back and side, and I cover the top with plexi (small gap for some air to get in and for bubbler typing to get out).


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Sometimes it takes a while for the tank to settle.

How big is the gap, it may seem small but have enough of an effect that it is draining the humidity - particularly if you're house humidity is on the drier side.

Also worth mentioning to calibrate your humidity gauge, it may be out.

aussieJJDude wrote:Sometimes it takes a while for the tank to settle.

How big is the gap, it may seem small but have enough of an effect that it is draining the humidity - particularly if you're house humidity is on the drier side.

Also worth mentioning to calibrate your humidity gauge, it may be out.

Its about a half inch gap. The plexi is actually sagging so I’m going to replace that with glass as soon as I can. I’ll recalibrate the hygrometer as well but I know my crabs haven’t been as active as they usually are (which isn’t much but they do roam around at night).


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aussieJJDude wrote: ↑

Sat Mar 09, 2019 7:12 pm

Sometimes it takes a while for the tank to settle.

How big is the gap, it may seem small but have enough of an effect that it is draining the humidity - particularly if you're house humidity is on the drier side.

Also worth mentioning to calibrate your humidity gauge, it may be out.

I second this. I'm only working with a 10 gallon aquarium & initially had problems keeping the humidity up. I had a small part of the screen lid not covered with plastic wrap. I thought it was no biggie and the air flow would probably be good/keep me aware of any stinkiness. Opted to cover it up to see how the humidity was effected & pretty instantly was rewarded with stable humidity in the 80-85% range.

Maybe adding a moss pit would help you too though, if you don't already have one.

craboloading wrote: I second this. I'm only working with a 10 gallon aquarium & initially had problems keeping the humidity up. I had a small part of the screen lid not covered with plastic wrap. I thought it was no biggie and the air flow would probably be good/keep me aware of any stinkiness. Opted to cover it up to see how the humidity was effected & pretty instantly was rewarded with stable humidity in the 80-85% range.

Maybe adding a moss pit would help you too though, if you don't already have one.

I do have a moss pit, but I don’t wet my moss at all. Do you?

Also, I completely forgot about Saran Wrap! Going to try this out tomorrow to see if that will help. Thanks!


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Wetting the moss may help give a boost to the humidity, I would suggest adding some water to the moss and seeing if that helps.... (plus my crabs really enjoy moist moss!)

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Maybe when you replace your plexi, get it to cover your whole tank with no gaps. My glass canopy lid covers the whole tank snuggly and my humidity never drops below 80*. It still fluctuates with the weather and if I open it, it can drop but having the whole lid covered really helps me.

RiNiKy506 wrote:Maybe when you replace your plexi, get it to cover your whole tank with no gaps. My glass canopy lid covers the whole tank snuggly and my humidity never drops below 80*. It still fluctuates with the weather and if I open it, it can drop but having the whole lid covered really helps me.

I have a split top, so I’d need two equal pieces to cover it or to remove the plastic divider at the top. Two pieces would be easier.

Also, the Saran Wrap idea makes me a bit nervous being that it would be under a light (Heat) source. I would think it would melt the plastic immediately.


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Yes the light will melt the plastic. What kind of lid do you have? Maybe try to find a mesh lid that fits and cover that with plexiglass, taping it down. Also a heat lamp will dry out the humidity.

This seems a bit silly but I will ask. What are the temperature readings inside your tank? Do you have the gaps at the front or rear of the tank? I think it could be from having too high of a temperature.

If your tank is warm 84 and up? If so the warmer temperature may be dropping the humidity. Perhaps lowering the temp inside the tank may be the fix. Possibly even changing the direction of the gaps in the lids may help. Both gaps to the front or back, or maybe the left side to the front and right to the back, maybe the opposite may be enough to change the airflow and balance things out too.

I had a similar humidity issue in the 20 I just built too. I was trying to keep the tank around 84-85 degrees. I was keeping a towel on the lid as a little extra insulation. Doing so was dropping my humidity to similar range as you. Once I removed the towel my temperature dropped you 79-81 and my humidity increased to about the same. Since I removed the towel a few weeks ago everything has been great!

Like aussieJJDude stated damp moss helps with humidity. I try to keep my moss pits away from the heat sources as much as possible to prevent them from drying out quickly. I still need to mist them every few days to keep the moss from getting crunchy.

LeeHasCrabs wrote: I have a split top, so I’d need two equal pieces to cover it or to remove the plastic divider at the top. Two pieces would be easier.

Also, the Saran Wrap idea makes me a bit nervous being that it would be under a light (Heat) source. I would think it would melt the plastic immediately.


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I forgot about the split top, you reminded me! I quoted you in the other thread too. Lol! I’m going to be needing the same type of thing, double canopy top. I hope it’s the same fit as I have now, except just with the two glass canopy tops. @Motorcrab or @Xenocrab you guys have the split top? How does that work, do you have double glass tops?


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We used the glass lid from our 20 Tall for one side of the 55. Then we bought another one for the other side. Then when I stared up the 20 again I had to buy another one.

Usually the glass lids will fit several sizes. The box will usually be labeled for what they fit. The two lids on my 75 are about 1/2" wider than they should be. I guess slight variations between different manufacturers. I would still double check and measure your tank just to be safe.

I have a large piece of glass here that if I cut in half will fit both sides perfectly. I’m gonna head to Ace in the AM to buy a glass cutter. I’m hoping that will help.

My temps are reading mid-70s for humidity and high 70s to low 80s for temp. I did dampen the moss tonight too. Although, my crabs like their moss dry, so we’ll see.


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I wet my moss pit last night and this morning my humidity is up to 81%. Gonna replace the plexi with glass today so we’ll see how that goes. But overnight my plexi didn’t warp, so I’m also factoring that into the bump in humidity.




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So I had a large piece of glass here that I cut in two for each side of my 50. Taped the edges so they wouldn’t cut me and also made little hinges out of the masking tape. So far after wetting my moss yesterday my humidity is at 82%. I’m happy with that if it stays around there. I think the crabs will be too.




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