Fiddler Crabs?

Do you mean you bought a snail instead of a filter? If so, yes, that is bad. A filter brings the tank water to a sponge and removes debris and detritus, then sends the cleaned water back into the tank, also adding oxygen.

A snail eats leftover food (which the fiddlers will get before it does) and pieces of dead plants (which there are none of). And it creates waste.

There are major differences between the 2.

The higher salinity level required for brackish may hurt or kill your snail. Leave the mystery snail in a full fresh tank, and put the fiddlers in a brackish tank. They'll also need access to fresh water, and land.

They may be able to live underwater for a few weeks maybe even a couple of months, but they will have a shorter life. Whatever they might tell you at the store purchase marine salt instead of aquarium salt. They need a species only tank as they may eat any other tank mates not of their species. Below is a list of items you'll need for their setup.

tank (5g minimum)
heater (keep the water around 75-82F)
filter (you can purchase a canister filter)
sand (some have said pool filter sand or play sand is ok)
marine salt (not aquarium salt)
hydrometer (used to measure salinity of water)
thermometer
food (crab food with calcium helps them when molting)
water dechlorinator
secure lid (they have been know to wonder out of their homes)
decorations (they are optional, but make the tank look good)

When purchasing your fiddler crabs look for ones that are active. Dont house to many males in one setup or they will fight till only one is left. Males have a one large claw and a small claw. Females have two small claws. They like to climb so make sure that your lid on your aquarium is secure. You should setup the tank before you purchase the crabs. And I've been told that you have to wait at least 3-4 days after mixing the salt and adding it to the tank before you can get your crabs. The crabs will molt every now and then, but dont remove the molt as they will eat it for calcium. If it is not eaten after a few days you then can remove it. You can feed them ghost shrimp as they have calcium, but some have told me that their crabs leave them alone. You'll have to do water changes also.

You'll need a hydrometer to calculate salinity levels of water. 1.010 should be good.

It kinda depends. Each time I mix up water for my crabs, I have to be careful because it's easy to over salt the water. I hate wasting expensive marine salt...