Sounds like you may have to make a food that they will eat.
Have you tried any of these:
1) The recipe here at Loaches. You can add meds to it. You do have to know how much your fish eat and how much meds per day you want to give them.
2) Mix a slurry of whatever they like and smear it on some rocks. Dry the mess. Put the rocks in the tank. I have heard of doing this with flake or pellet foods. I would assume it would work for almost any food that you could get into a slurry. The problem is adding the meds. I do not know if they will still be active once they have been wet then dried.
Maybe best to order the other medicine.
38 tanks, 2 ponds over 4000 liters of water to keep clean and fresh.
Happy fish keeping!