Although that is probably a great long term food. You may want to offer something meatier just to get some food in it.
You may want to try small chunks of Rod's food original blend or other meaty mixture.
I say this because often newly bought fish are on the thin side and it's body will need energy to fend off the ich.
Hopefully it will go for something quickly for you. I would probably pass on frozen brine unless you can't find anything else. As it is said that there is little nutrition in that food.
Good luck!
---------- Post added at 11:19 AM ---------- Previous post was at 11:17 AM ----------
Call the store, they may have other advise as well.