There are probably a few things that you could do to upgrade your tank, to improve their conditions.
HCA recommends substrate to be at least 6 inches, or three times as deep as your largest crab - whichever is deeper wins. Many of us use a mix of 5 parts playsand and 1 part EE (coco fiber). Other mixes have been used successfully, but this ratio helps with humidity and holds moisture well, as well as being cheaper than all EE and easier to maintain. It needs to be wet enough to hold it's shape when the crabs dig and tunnel, but not so wet that water pools in the bottom.
Commercial foods are not safe for crabs long term, even though they are marketed for them. Crabs can eat many of the same things that we do. They need calcium and protein daily. Be sure to read the nutrition and safe/ unsafe food lists.
HCA recommends at least 3-5 extra shells per crab, to help prevent fighting over shells. There are proper and improper shells to buy, be sure to read the shells care sheet.
Here is the link to the care sheets. If you have more questions, feel free to ask!
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"If there are no dogs in Heaven, then when I die I want to go where they went." -Will Rogers