If it's a land-based hermit crab, you can care for it in a 10-gallon tank with a sand base, some hiding spots, some places to crawl around in, and a sponge soaked in water. To feed it, use pellets that are made specifically for hermit crabs. If it's a water-based crab, you can put it in your aquarium as a scavenger to hep keep the tank free of food that would otherwise sit around and cause water quality issues.