Pink fish

Question
Hi. I have a fantail who recently had fun rot and was treated with erythromycin successfully. Then a few weeks later I noticed a red streak on his fin so treated a second time to be sure it wasn't coming back. Now he seems to be slowly turning a pinkish colour. The water is good although obviously all the natural bacteria will have been killed by the antibiotics.
Is he dying or what is going on?

Many thanks!

Answer
Hi Ciara,

If your fantail has healed already, I would suggest a complete water change. You see, erythromycin belongs in a group of drugs called macrolide antibiotics. Macrolide antibiotics slow the growth of, and kills sensitive bacteria by reducing the production of important proteins needed by the bacteria to survive. However, all fish need bacteria in the water to survive; not all bacteria in the water is harmful to fish. In fact, some are actually beneficial to their health.

I hope this helps!
-Sam