White Lump On Female Dwarf Rainbow Fin.

[harry246]

I had two female dwarf rainbows in a community tank and one male. 
 
One of the females had fin rot. I treated it with melafix and everything appeared normal again. The fin looked healthy again.
 
Then, one of my females disappeared. I believe it got into a fight with the male as some of the males fins are now missing. Local fish shop said they will take time to grow back. 
 
Anyway, now the one remaining female which I believe is the same female I treated for fin rot, has a white lump on it's fin. 
 
I posted this in emergencies as i'm not sure if it is one. The female is eating and swimming properly and acting normally. 
 
Please see the pictures provided below.
 
 

 
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If anyone could tell me what is it and how it's curable. If you could also let me know if I need to quarantine her away from all of my other fish for treatment that would be great. 
 
I'm going to my local fish shop tomorrow to ask them about it as well.
 
Thanks.

 

[laina45]

I have rainbows too. I lost my dwarf about a month ago, had a darker lump between his dorsal. 
Now I have an australian rainbow with the same discolored spot along his back, and also a raised lump by his fin.
It looks a lot like yours. I am not sure of the cause, or the treatment unfortunately. 
Here is a link to my post, maybe my experience can help you decide what to try, if you agree it is the same thing, but it sure looks like it to me. 
http/www.fishforums.net/index.php?/topic/419562-disease-in-tank-help-1-gone-already/