I am with Giau (sorry for all the losses). I'd still be looking for some kind of connection. Maybe the in the system existing ich is attacking the the stressed fish (being new as being the factor for being stressed) or something. I have no clue.
I am just worried if there is a connection it may affect any other future new additions.

About the food. Thawing and refreezing damages the cell walls which leads to nutrient loss. Surely not a cause of death but perhaps not helping with recovery either if the food has a lower nutrientional value than it normally wood?