How to Set up a Fiddler Crab Tank

How to Set up a Fiddler Crab Tank

Fiddler crabs (Uca sp.) are one of the most popular semi-terrestrial crabs in our hobby. So, if you are thinking about setting up the tank for these interesting crabs, you have come to the right place.

Fiddler crab tank should have land and water parts. The substrate should be deep enough to borrow with easy access to brackish water.

Sounds pretty simple, isn’t it? I need to start off by saying that Fiddler crabs are hardy and llow-maintenance pets. It means that they do not require a fancy setup or extra aquarium equipment.

In this article, I will be talking about how to set up the ideal environment for your Fiddler crab. It will be from A to Z description with lots of tips.

Budget set up for your Fiddler crabs (links to check the price on Amazon)

Step 1: Choosing Your Fiddler Crab Tank

In my opinion, this is one of the most important things and you need to get right from the start.

Just because Fiddler crabs are relatively small crabs (slightly over 2 inches (5 cm) across the leg span) does NOT mean they can be kept in tiny tanks.

Why does it matter so much?

  1. It is easier to maintain water quality in bigger tanks because the larger water volume dilutes the waste products. The bigger the tank, the easier it will be to look after, especially, if you are planning to keep several Fiddler crabs.
  2. It can be very difficult to create a proper hardscape for your crabs in small tanks.
  3. Even though Fiddler crabs are social and live in colonies, they still have very complex behavior. In small tanks, these crabs will definitely arm-wrestle and fight each other more often.
  4. In addition, in a small tank, it can be difficult to create a natural environment in which your crabs will thrive.

Therefore, when sizing an aquarium for a Fiddler crab, I would recommend starting out with a tank size of 10 gallons (40 liters) for 3-5 crabs.

Of course, whenever it is possible, always opt for a bigger size. After all, you want to own happy and healthy Fiddler crabs – you want them to thrive, not survive!

Note: Even if you decide to keep only 1 or 2 crabs, you will still want to start out with 10 gallons.

1.1. Shape of the Tank. Length Over Height

Empty shrimp tankIf there is a choice, long tanks are better than tall tanks of the same size for keeping Fiddler crabs.

Unlike fish, these crabs need mostly a place to crawl from side to side, not up and down.

Step 2: Cleaning, and Testing Fiddler Crab Tank

DO NOT underestimate this step!

shrimp tank Hydrogen PeroxideAfter setting up your crab tank, there is nothing worse than finding out that the tank is leaking or there is some kind of toxic residue.

  • Cleaning

First of all, you need to clean the tank. You can use bleach, vinegar, or hydrogen peroxide for that.
You can spray them on the walls, and let it sit for 10 – 20 minutes. Next, wash it off properly (do it at least twice to remove everything). DO NOT use soap, it can leave a residue in the tank.

  • Leaking Test

To make sure that the tank is not damaged. Put some paper underneath it and fill it with water for 1 day.

If you do not see any wet spots on the paper, the tank is safe to use and you are good to go.

For more information, you can also read “How to Clean and Disinfect Used Aquariums and Accessories”.

Step 3: Deciding Land vs Water Ratio

Fiddler crabs are neither aquatic nor terrestrial crabs; they are semi-terrestrial crabs.

In nature, these crabs are often found living on tropical intertidal mud and/or sand flats (mangrove habitats). It means that they have a clear preference for terrestrial areas.

So, it is not recommended to keep them in tanks where they can only climb and sit on rocks and driftwood.

Another important factor is that Fiddler crabs have lungs, so they have to come to the surface to breathe. That is why the land vs water ratio for the Fiddler crab setups should be around 80:20 or 90:10.

We can do that in two ways:

  • Using a water bowl.
  • Using a filter (classic setup).

3.1. Water Bowl

The good thing about the water bowl is that it is easy and simple. At the same time, this method has a number of restrictions and there are some important rules to follow:

  1. The bowl should be at least 3 – 4 niches inches deep.
  2. It should be big enough to fit at least half of your crabs in it at the same time.
  3. It is also important to provide a variety of surfaces for them to get out of the water. It can be rocks, sticks, etc.
  4. Without filtration, you will have to replace this water every 2-3 days.

Pros and Cons of Water Bowls

Pros: Cons:
All water changes will be very easy and simple. Water bowls usually do not look great in the setup unless you know how to decorate and hide things.
There will be no need to use any filters to maintain cleanliness and prevent toxicity!
Frequent water changes. You will have to replace it every 2-3 days.

3.2. Classic Setup (Filter and Pump)

Instead of a water bowl, you will need to use an aquarium filter to the Fiddler crab habitat to keep the water clean. Adding water aeration will also improve water quality thus increase their lifespan.

An internal filter will be the best choice because it does not have intake tubes that your crabs can use to escape.

At the same time, you can use whatever filter you want. Just keep in mind that sponge and hang on the back filters do not work well with lowered water levels. 

Option: Use egg crates. Place a mesh screen over egg crates and put the substrate on top of that.
Tip: You can also use a filter pump under this construction.

Pros and Cons of Classic Setup

Pros: Cons:
Less frequent water changes More expensive
Provide stable water parameters Requires regular cleaning and maintenance.
  Fiddler carbs can use cords and intake tubes to escape.

Step 4: Preparing and Adding Substrate to Fiddler Crab Tank

In the wild, Fiddler crabs love to dig, stir and create tunnels in the substrate. They do that in search of food and in order to escape from predators and avoid desiccation due to high temperatures.

We do not want to deprive them of what they would experience in their natural habitats. So, in the tank, they should not be denied the ability to act as they used to.

Substrate plays 2 important roles in Fiddler crab tank setup:

  • Hiding places. Let’s them excavate their own dens.
  • Humidity. Helps to maintain the humidity level in the tank.

4.1. Substrate Types

shrimp tank Carib Sea Super Natural SunsetDepending on our tank setup we can use:

  • soil (mud),
  • sand,
  • coconut fiber. 

Small gravel can be used but only if you provide them with some burrowing areas. In all other cases, gravel substrates are a no-no! 

Note: I have heard multiple times people saying that gravel can break off the tips of Fiddler crab’s legs. It is not true. It looks like somebody said it once and now almost everybody repeats this nonsense. It is always and only about burrowing.

4.2 Substrate Consistency

The substrate in the Fiddler crab tank should always be kept moist and have a ‘sandcastle’ consistency. It means that it should hold its shape when you squeeze it.

Interesting fact: Some species will even cease feeding and seal themselves inside their burrows when the surface soil is no longer saturated.

How do you know that you have ‘sandcastle’ consistency?

  • Take a pencil.
  • Stick it all the way down the substrate and pull back up.
  • The tunnel should not collapse.

4.3. Substrate Depth

How Deep should be the Substrate in Fiddler Crab Setup?

According to multiple studies and observations, Fiddler crabs usually create nearly vertical and straight burrows. In general, their length ranges from 4 – 8 inches (10 – 20 cm).

Therefore, ideally, your substrate should be at least 4 inches deep.

4.5. Fiddler Crabs in a Bare Bottom Tank

I have seen a few people keeping their crabs in bare bottom tanks.

Is it possible? Yes, it is.
Is it good? Absolutely not!

As I have already said, you will lose out on watching them act as they normally would in their natural habitats. It will also make them very unhappy and stressed.

If it continues over time it may weaken their immune system, making them more vulnerable to diseases.

Therefore, unless it is a quarantine tank, I would never recommend keeping Fiddler crabs in a bare-bottom tank. 

4.6. Preparing Substrate to Fiddler Crab Tank

It is always recommended to clean and rinse substrate to get rid of any debris or dust that might have been accumulated on it. This applies especially to sand and small gravel.

If you forget to rinse your sand, it may turn your tank into a cloudy mess that will not settle for a long time!

Therefore, even if you see on the bag that it is promoted as “pre-rinsed” – take it with a pinch of salt.

How to Prepare Sand for Fiddler Crab Tank

  1. Place it into a bucket and spray the sand with a hose.
  2. The excess water will run out of the bucket and the water will be cloudy and dirty initially.
  3. Keep rinsing. You will notice that the water will run clearer.
  4. If you think that sand is clean enough – rinse it again for a few more minutes!

How to Prepare Gravel for Fiddler Crab Tank

  1. Put the gravel into the bucket.
  2. Fill the bucket with tap water. The water should completely cover the gravel.
  3. Stir and move the gravel around (use a stick if necessary).
  4. Leave it in the bucket for 5 – 10 minutes. It will soften up any dust and debris that may be on the gravel.
  5. Stir and move the gravel around one more time.

4.7 Adding Substrate to Fiddler Crab Tank

Once the substrate is clean put it in the tank and spread it evenly over the bottom of the tank.

To improve visual perception, aquarists usually do a little slant to give the aquarium a little more depth. Make it a little higher in the back and lower in the front. Because of this trick, the size of your tank will look larger. In addition, it will be easier to look at your Fiddler crabs.

Tip: Use any unnecessary credit card to flatten out the top.

Step 5: Preparing and Adding Water to Fiddler Crab Tank

Fiddles crabs are brackish water (mixture of freshwater and saltwater) animals.

Generally, the specific gravity (SG) of brackish water can vary between 1.005-1.021. However, to measure it, we have to use a refractometer (link to check the price on Amazon).

Luckily, Fiddler crabs are very hardy creatures. Uca species can osmoregulate powerfully over a wide salinity range. That is because, in their natural habitat (mangroves), fluctuations in salinity are very large. 

Therefore, we can prepare brackish water even without a refractometer.

Sea salt productsHow to prepare brackish water:

  1. Take Sea Salt (link to check the price on Amazon).
  2. Mix 5 tablespoons of sea salt per 1 – 1.5 gallons (4 – 6 liters) of water.

Do not worry if you add a little bit too much or a little bit too little.

Important: Do not ever use simple aquarium salt or table salt!

Step 6: Adding Decorations and Plants

Some people do not add many decorations to the tank because they do not easily see their crabs in the tank.

These people make a mistake and do more harm to their Fiddler crabs!

Hiding spots should be installed in the tank so that they can have their individual spaces. It will minimize the stress level and improves their survival rate. If the tank has a lot of secluded places, even in crowded tanks, it will be rare for any specimens to lose a claw.

Some examples:

  • ceramic or plastic flower pots,
  • flat stones and rocks,
  • driftwood,
  • sticks,
  • pieces of bark,
  • plastic balls,
  • plastic tunnels,
  • PVC pipes,
  • crab huts,
  • fake plants,
  • bricks, etc.

Related article:

Important: Before adding any decorations in the Fiddler crabitat, it will be better to quarantine or cure them. Make sure that everything that you put in your tank is safe.

Here is a word of caution for those who have thought about adding outside rocks, etc. to their tanks.

Think twice before taking a random rock or something from outside and putting it into your tank as a decoration. These items may contain bacteria that can affect your Fiddler crabs or leach unwanted substances into your tank.

For example, boiling the rocks will get rid of any nasty residents that may enter your tank. At the same time, the gas created from boiling the rock may be poisonous to humans depending on what is actually on the rock.

Therefore, be very careful.

 Step 7: Installing Aquarium Light

No special requirements. Fiddler crabs will not be bothered about the kind of light you use.

So, the lighting is mostly up to your taste and how well you want your tank to be illuminated.

Step 8: Cycling Your Fidler Crab Tank (Optional)

Whether or not you need to cycle the tank depends on step 3.

  • You do not need to do that if you use water bowls.
  • You need to do it if you use filters.

To put things simply, Fiddler crabs create ammonia when they produce waste (poop or pee). Ammonia is toxic to all animals and should be dealt with either through:

  • Full water change (water bowl method)
  • Nitrogen cycle (classic method).

During the cycling process, the beneficial bacteria will break down ammonia to nitrites and nitrite to nitrate.

Once the cycle is complete (you should not have ammonia or nitrites), it is time to add the Fiddler crabs.

In Conclusion

Setting up a Fiddler tank is not difficult and can be easily done even by beginners.

With a bit of research and investment of time and resources, anyone can recreate the ideal crabitat for these animals to thrive in their new home. If you want to make sure you do things right so you can have a happy, healthy pet.

If you still have some questions, feel free to ask me.

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2 thoughts on “How to Set up a Fiddler Crab Tank

  1. Hello,

    We are using the water dish method for our fiddler crabs. How often do we need to replace the sand in the tank? Do we just scrap the top layer off every week to remove any waste the crab produces?

    Thanks for the help!

    1. Hi Christine,
      Although it is not necessary to replace the sand frequently, it is advised to do so every 6 to 12 months to avoid the accumulation of bacteria and waste.
      Best regards,

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