Red clawed crabs are not truely freshwater, they live in estuaries that contain brackish water so that is their proffered environment. The may survive in freshwater but will certainly not thrive. When I kept mine I fed them high protein food and tablet food.
No problem. When I first had mine there was barely any info about them so I kept them in freshwater not realising and they usually died within a month or two encore I realised something was going wrong. You probably already know but don't forget to give them some dry land above the water.
actually they are brackish, and not remotely freshwater.
they also live on land and not in the water.
though they sleep under sand as the tide comes in.
so, ideally, you need land with a bit of water, not the other way round.
as for food?
they have a basic omnivores diet.
so, roughly 25% protein, is all that's needed.
fish flake, algae wafers and catfish pellets go down well.
also adding some veg that is a little bit past its best works too.