Well, maybe if you dont know anything about hermit crabs, you shouldnt take them from the beach. Did you wash the sand from the beach? And you will have to change the water every 3-4 days. As mentioned, Red Sea Salt is the best so far. And dont you ever you table salt cause they will sure die.

Speaking of food, it is better if you provide some fresh food. I dont think hermit crabs will eat grass. If you consider preparing fresh food a hassel, then you should release them back to the beach. And try not to feed too much cheap fish food, cause sometimes the quality is not so good and there are certain preservatives in the food.