Fiddler Crabs

The petsmart where I'll be gettingthem has them in a tank with half water and half gravel. They look fine. They have a waterfall in the back of the tank. They also have a food dish in the tank. Everytime I visit they have fiddler crabs. Are they ok at a temp of 79-80F? Are they ok with under the tank heaters?
 
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If I set my tank up as shown above, I'll save money and I can have my fiddler crabs by this weekend. All Im gonna have to buy is a hydrometer, sea salt, and a test kit. Im gonna add my heater to the bottom of the tank in the back. It's at 80F is that ok? I also have some small river stones for the tank. I have it setup in my room. Its just to give me a look at how it will be. I thought of a great way to make burrows in gravel for the fiddlers, but I dont know about it. You can get a regular plastic water bottle and cut of half of it. Then you can arrange it in the gravel for the fiddlers.

*When I get the hydrometer, should I get the one that looks like a thermometer or the one you fill with water?
 
I don't really get the half-cut bottle idea.

You can get playsand, which is pretty cheap.
 
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