While community tanks are interesting to look at and fun to keep, you have to choose the right combination of creatures to house together. Fiddler crabs are small, active crustaceans who live in brackish swamps in the wild. Goldfish are coldwater fish that are generally easy for a beginner fishkeeper to maintain. Since they require very different conditions, it's not possible to keep the two together.
While goldfish and fiddler crabs are both relatively peaceful, they can't live together as they have different environmental needs. Goldfish are coldwater fish. As they prefer temperatures close to 60 degrees Fahrenheit, their tanks generally don't need heating. Fiddler crabs, on the other hand, require their water to be heated to between 68 and 77 F. Goldfish are freshwater fish, so their aquarium needs to be free of salt. Since fiddler crabs come from brackish environments, they need roughly 1 tablespoon of aquarium salt added to each gallon of their water. They also need areas of dry land in their setup, on which they can rest in the open air.
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