Help with treating tetra with cotton wool mouth

[Conrad]

One of my neon tetra has developed a growth on his lip and I've moved him to my 13 litre hospital tank. This happened to a different tetra not long after I started up my tank but he recovered well from it. Both times I have treated the fish with interpet anti fungus and finrot. I have followed step by step instructions but on both occasions after adding the medicine to the tank I've witnessed the fish vomiting. Am I doing something wrong because I'm very worried about using this product in future if it causes this problem to my fish.