Do you mean for their diet or for their water?
For the water it depends on your KH and GH and pH.
I use a Cuttlebone in the water - the inverts can nom on it any time they want to.
Also for diet I use a food specifically for the inverts with extra minerals for health and molting. Like a Shrimp food or Crab Cuisine from Hikari.
Also because I have 0 KH in my water I use other water additives for my water, like marine calcium and epsom salt.