Most of the time you will not see any existing symptoms of ich (depending on stage of c. irritans by the time you are looking at the fish in the LFS
is definitely important, especially that you have a pre-existing BHT in your tank.
Since you don't have any crustaceans and inverts that will be missed, you might want to do a hypo in your display. This is definitely not advisable. However, you have specimen in your tank that most likely have the ich as well. Also, your sand and rocks will have c. irritans in there as well.
At this point you will have to erradicate it in your main tank. You can either (a) do a QT
of all of your fish and treat them there, or (b) if you don't have a QT
and it is at a desperate condition, do a hypo in your main. Again, this is not advisable. I am just talking about your situation wherein you don't have corals and anemone, crustaceans, and in dire need of saving your BHT from sudden death.
PS: I did it in my main tank. First pic is before. Second pic is during. Third pic is after. I lost a red tail file fish, and a white spotted puffer. I donated my porcupine puffer, damsels, and the cowfish.