Setting up your crab environment is an interesting endevour. The old fish tank will not be the right appraoch. You will need a Reef Tank to pull this off. If you do not provide a good ecosystem for your crabs they will not thrive and may dis so heads up, you need to set up right. Crabs need rocks to clinb on that rise above the water. Salt water is the rule of the day and your cabs will need the right feed to survive. Just read up and be prepared. Dont let the details change you desire to build that Reef Tank. https://ideamensch.com/joe-olujic/ is a good place to start that set up.