It has come to Rack’s attention that there is confusion out there on what to feed hermit crabs. So he’s decided to give you some guidance.
If you don’t feed your hermit crabs protein regularly (preferably daily), they will become aggressive, fight each other, and possibly cannibalize their tankmates to get this necessary macronutrient. Protein can be as easy as dried shrimp/krill/crickets/earthworms/water flies/etc. found in the fish and turtle sections of pet stores. Ingredient lists should only have the item in it (as in, a bottle of dried shrimp should have the ingredient of shrimp, and nothing else). Alternatively, you can give them bits of whatever meat you’re eating for dinner, but make sure to get it BEFORE you season it, because some seasonings are toxic to crabs.
Calcium is another nutrient they will possibly kill for. They can get calcium off of their shells (they can and will chip their shells to make them fit their eyestalks better, for instance, and can eat that), their food shells (if you use them), or the sand that you might use in your tank. You can also rinse and microwave/bake eggshells and keep them in the tank, provide a cuttlebone, or provide broken shells for them to chip away at.
Everything else is more of a ‘nice to have’ sort of food. Experiment (but check its safety first at the Epicurean Hermit) with what your crabs like. Mine prefer dark or bright fruits and veggies. Spinach, kale, bell peppers, and tomatoes have all been favorites; crabs definitely do have a bit of a sweet tooth and like getting mushy (but not moldy) fruits. Parsley and cilantro are also hits, as is avocado (just give them a piece of skin and they’ll scrape out the fatty goodness that your spoon couldn’t remove).
The key with crabs is to offer a variety. They have been found to NOT eat foods they’ve eaten recently in favor of novel items. This adaptation keeps them from having inbalanced diets in the wild, thus (hopefully) allowing them the nutrition they need to successfully molt and reproduce. So on any given day in my tank, they have a minimum of two types of protein (one, like shrimp, that has a chitinous shell, and one, like fish, that doesn’t), one calcium (aside from their cuttlebone and eggshells that are always in the tank), a nut, a fruit, a veggie, a wood or leaf (they really like mangrove meal and white oak bark), a seaweed or algae, and a treat (organic earthworm castings, bee pollen, or astaxanthin powder, which makes their color darken to the lovely purple shade modeled here by Rack). Plus whatever fresh stuff I have on hand, although that’s optional.
I buy my dried foods from the Hermit Crab Addiction Store and have always had good luck with her. Everything she sells is confirmed to be non-toxic to crabs.
TL;DR: give your crabs protein and calcium every day, and experiment with all kinds of stuff!