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I'd get rid of the fish pebbles get sand, and have fresh water and something with salt water for them because they are salt water creatures. A 3 gallon tank is a bit small for 6 but the pebbles will break their legs. Good luck
I have had fiddler crabs for some time now and a lot of the info you have is great. I also have moon crabs and hermit crabs and before I became a mini pet shop due to my sons I had them all in one tank and they got along just fine. I have them all in different tanks now, they all have different attitudes. My fiddlers like to climb and swing craziest bleep I've seen and I've seen crazy things.