There is nothing you can medicate the food with to cure Crypt....so what are you adding to it? If the fish isn't eating medicated food, just stop medicating it, you want the fish eating, that is the No. 1 component of a healthy fish.
Besides that, do the responsible thing, and set up a quarantine tank, they are cheap and very easy to set up. It doesn't have to be a glass tank, just anything that holds water.
Given that the parasite is in the tank, you really should pull everyone out and actively treat them with copper, hypo, whatever while leaving the tank fallow for 5-6 weeks. It's going to easier then having to constantly deal with the parasite every time you add a fish....trust me....I lost a good deal of fish before I started QT'ing.