Take the crab back to the store, or better yet, set up another tank for it, as the store clearly kmnow nothing about these crabs.
They need at least mildly brackish water (that means part sea water, made with marine salt mix) and are amphibious land crabs, they use more land than water.
Keep it in freshwater, and it's claw wont regenerate, and it will die one of the next times it sheds its skin.
And yes, there is also a high chance it will eat your neons.
For one red claw crab, a 1'x1'x1' tank should suffice, with at least half of it a land area, and the water brackish with a small filter and heater.