been thinking of making a 20L in to a crab tank
i know they have to have some land areas
any one keep these?

I have had this once, what i did was use and old 25 gallon aqurium fill it with 5 bags of 2 pound sand untill it made a good base then I dug a hole in the sad and put a medium sized containor fill it with water put a tiki hut and did some landscaping and some aqua scaping till it looked cool it fit like the most i had in there were 6 fiddlers crabs

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I have kept fiddler crabs on and off all of my life (and I am old)
They are very common in NY and in the summer I always collect a few.
You need to house them in a tank that had a good deal of land in it. They only go into the water to wet their gills but spend most of their time on land.
They are also cannibals and will fight each other, especially males.
I have a 5 gallon tank where I built a partition out of cement so more than half the tank is sand and the rest is a couple of inches of sand. I have a tiny pump in the water that pumps water onto the sand in a corner. The water makes it's way through the sand and back to the water. The crabs dig in the sand to escape each other. You can feed them any meaty food like clams or worms. They don't need lights

I am setting up a 15g for fiddlers.

ive always liked em and kept terrariums for years with running water via duetto filters and want to try these onery creatures.

Helps that my 5yo and 4yo kids want in on this too, easier to persuade mom into the idea.

i was thinking using one of the whisper intank filters on one side

I had one also many years ago. I really enjoyed it. I took a piece of glass and siliconed it into the tank, to make a divider of water area and sand/land area.
I had mangroves growing in it and a nice warm and bright light on a timer.
The crabs would warm up and by the middle of the day they were fully energized. They would smack each other around with their claws at times, and you would hear them hitting the glass in the tank.. it was a riot.
Very enjoyable tank and you will surely have a nice time keeping it.