Hi Christoph,
Humblefish is correct I did bind powdered medication with fish oil to pellet food and that works well if your fish is eating well and fast.
I tried the same as ngoodermuth, using gel type food but unfortunalty with the same results. after this experience I did try to feed my rabbit fish some gel type food, of course without meds, but not even this guy was willing to eat it and this guy usually eats everthing and a lot of it, I guess that speaks for the taste of this gel type food.
recently I spoke to a guy who is in his 60' now and was in the ornamental fish industry since his boyhood. They used and use medications over the counter, grind them with mortar and pestle, dissolve the fine powder in water (RO), soak fish food in this mixture and then, to make it tasty, coat it with liquid garlic. His preference is to use freeze dried brine shrimp as the brine shrimps are bone dry and do absorb the medicated water well. On the other hand this is mostly done for internal infections and external should be treated with medicated water.
I had a maroon clowfish with a mild fin rot which turned out to be a little nightmare and it took me 3 weeks to stop the fin rot. Which indicates and confirms that some bacteria are nasty and or the antibiotics are just working slow.
cheers
Soren