Remember that hermit crabs are beach comers and eat what they find there. A good crab pellet food should be bought and fed to insure they get vitamins they need. They also like fruits and vegetables. Grapes are a favorite of mine and he also likes peanut butter. If possible feeding them in a half clam shell is good as it is most like their ocean origin.
Plastic is alright but never use metal, as it is harmful to their health.
I also put water in a half clam shell because of the calcium in the shell which is good for the crab. On a daily basis I spray my crab with water inside his shell to make sure he does not dry out. His water and food should be changed daily so that he will not eat food that has decayed. His sand should be changed or boiled monthly. I use a screen lid and rest the light fixture on the screen.