I have a terrarium of Red-Devil Crabs.

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The terrarium is about 70% land, 30% water. I have pea gravel underneath the Exo Terra bedding, so that it isn't touching the water. There's no airstone or filter as it's about a liter of water and is only about an inch deep. They are terrestrial crabs, so the water is only for them to molt in, and occasionally drink from.

I keep a few aquariums, so I have equipment for water changes. What I don't know is if I should change the water in the crab's terrarium about the same frequency as I would for the aquariums, or less since the crabs are never really in it. I want to keep the water fresh for them, but if there's nothing in the water to cause it to go "bad" how often should I change it?

  • I don't have enough knowledge about these guys to really answer the question, but I can tell you that standing freshwater can go anaerobic with even a small amount of organic matter. Do you have any kind of airstone/filtration that keeps the water moving? And what's the volume of water?
    – toxotes
    Dec 20, 2013 at 17:39
  • No airstone or filter, it's about a liter of water, and it only sits about an inch deep. They are terrestrial crabs, so the water is only for them to molt in, and occasionally drink from.
    – Spidercat
    Dec 20, 2013 at 17:44
  • We used to use a tube to syphon the water out and into a bucket and then do the same to replace the water. Any long hose will work, probably will need at least 4 feet.
    – Chris
    Dec 20, 2013 at 19:51
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    or you can go creative and make a little fountain dripping over a cascade of rocks where you can know it gets filtered and aerated Dec 20, 2013 at 23:09
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    Agreed, without filtration or aeration, it's important to change the water every couple days just to ensure it doesn't go bad or host unwanted bacteria/critters. Not to mention any decaying organic matter in the water can cause unwanted reactions to occur. Dec 21, 2013 at 14:22

1 Answer 1


I've found that with a properly set up terrarium, the substrate shouldn't come into contact with the water, so the only decomposing matter in the water should be the crab's molted shells.

Since my terrarium is 30 gallons, and it's only used for molting, I've elected to change the water about once a month. I also put a pinch of aquarium salt into the water after the water change, and about halfway through the month until the next water change, to add minerals to the water that help the crabs molt.

In hindsight, I wish I had taken the time to build a miniature sump to take the water out of the tank and filter it back in. Otherwise I think putting a waterfall, or pool feature, like you would for poison dart frogs would be best. Just note that the crabs can't swim, so it's important for them to be able to climb out of the water, or they'll drown.

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