is growth on goldfish normal?

I successfully medicated my goldfish for whatever it was that they had, but
now my water has a pinkish color to it. Any ideas as to why? I used
tetracycline for medication and it initially turned my water color yellow,
but turned back to clear after I ran it through carbon. I've been running
the pinkish water through carbon and doing 25% every other day water change.
I did a 50% water change over the weekend and changed out the carbon, but
the pink color is still there.