Red Rainbow...

[Lonewolfblue]

I just noticed a single large white bump on the back of the rainbow. Will try to get a pic of it soon, but it is located right in front of the dorsal fin. I thought it might just be an injury, but seems to be getting bigger. Any ideas? I'll post a pic shortly.

Eit:

Here's pics......

 

[Devilishturtles]

Looks like an either benign or parasitic tumor, a genetic malady. There is nothing you can do for it...simply keep an eye on it to see if it goes away or gets larger. It's been my experience that they will pop up during times of stress and then go away again. It could be the start of a tumor that will continue to grow until you have to euthanize the fish.

HTH