Clownfish with redness near mouth

[Zirco]

A couple of weeks ago I bought two baby clownfish. They're the only two fish in a 150g tank. One has developed some redness around her mouth. It actually looks like its slightly internal, not right on the surface.

Any idea what this might be?

 

[Zirco]

I should mention that I first noticed it on the day that I added the first corals to the tank - two days ago.

 

[Zirco]

For what it's worth, it got slowly worse over the course of a week and one day she seemed genuinely distressed. I figured she was a goner but the next day she was acting better and a couple days after that the redness started to abate. Now she seems totally fine and redness is gone.

it's probably just a coincidence but she started to improve the day after I set up my skimmer.