Growth on astyanax bimculatus

[cichlidfish]

A while ago this fish developed a black growth on his side. It got bigger, and tunred pink. The fish was very healthy with no change in behavior. A few days ago I noticed the growth was practically gone. Looks like it's healing up.
My question is what was that? A tumor?

Then an now pics.

 

[celebrist]

No clue but that's gnarly

 

[Hendre]

Looks like a flesh wound to me?

 

[cichlidfish]

Looks like a flesh wound to me?

At first that is what I thought. Then the thing grew. I am missing an inbetween pic where that thing was bulging out. It was a huge growth. But something happened to it overnight. These have to be the hardiest or toughest fish. This female is 7". She was bitten by a p. Loisellei and I think he got the swim bladder. She started swimming funny, so I put her in the hospital tank. I only did water changes and she was back to normal in 2 weeks.