Please check out the Epicurean Hermit! This is a website that contains a safe food and unsafe foods list, and even some recipes for your hermit crabs!
Remember to only feed your hermit crabs organic food! Be sure to also check the inedible list so you know what they can't eat!
I have a big personal problem with commercial foods, although many people disagree. Many commercial foods contain substances and chemicals that are harmful to hermit crabs. I personally do not use them. I only feed my hermit crabs natural foods. I like to know what I'm feeding to them, and where it came from. If you decide to feed your hermit crabs only natural foods, only feed them organic!
Hermit crabs need water just like every other living creature on Earth. Hermit crabs need fresh and salt water to survive. Both their fresh and salt water should be treated with a water conditioner to make sure there are no metals or other chemicals that could harm them. Tap water is not suitable for hermit crabs unless it is treated. Do not use table salt to make salt water for your crabs. Instead, use pure sea salt which you can buy at your local grocery store. I prepare my water for my crabs by the gallon.
I store my food in small Gladware containers and Ziploc Freezer bags. I usually buy a few different types of vegetables and fruits, and chop the whole thing up and toss it in the freezer. If you get a heavy duty freezer bag, and make sure to get all of the air out of the bag, the fruits and veggies can last for months! I just break a piece off and toss it in the tat. I store my crushed eggshells in a small Gladware container after crushing them with my mortar and pestle. I do the same with cuttle bone (crush it up with my mortar and pestle, and store it in the small Gladware containers.)