My preference for medicating fish with digestive issues is through gel food like Repashy Bottom Scratcher. I pre-measure everything needed to make the gel food so that I can just dump the ingredients in. I boil the water needed, mix the gel powder into it, and wait for the steam to leave the food and to harden halfway, and then put the medication in, allowing it to cool to room temp for 15 minutes before putting in the fridge. I've had WC poly and CB poly take the Repashy Bottom Scratcher.
You can start off with something 3/4 teaspoon of pure epsom salt and 1/2 cup of distilled water mixed together as the liquid portion of the gel food. If no improvement, move on to using the same liquid solution, plus Seachem Metroplex & Seachem Focus (1:1 ratio) in the gel food.