Very good idea!
If you do want some foods that you can leave in the tank for a few days (I have a dry food dish that I will sometimes leave for a week or more) in addition to your fresh food which should be removed daily (and should still be provided daily), you can check out
http://keeponcrabbin.com/
Owned by a member here, Spidtat, who decided to start a line of hermie food himself. He also sells foods from Crabotanicals, a well established line of dried hermie foods.
I highly recommend any mixes that contain seaweed, as they should have seaweed daily, and it can be difficult to find more than spirulina and kelp in the grocery store, and the seaweed sheets at the pet store can get spendy.
Remember, they need that fresh food daily, but this is a nice supplement you can put in and leave for several days. Also remember that hermit crabs won't eat something they've eaten or smelled for 14hrs up to two weeks. I leave it in to give all my crabs (14) a chance to get some, since they don't all eat it the first day it's in.