Sick Guppy?

[~heidi~]

I was looking at my fish today and noticed that my favorite guppy has grown this yellowy "lump" on its back. It was not there yesterday and he is eating fine. I have separated him from the rest of the tank and am wondering how to treat this. 
Thank you very much for any help. I am very worried about him.
 
The rest of the fish in my tank look fine.
 
 

 

[Blitz]

Perhaps it is a cyst. Is it growing bigger and does it affect him on any of his abilities(swimming etc)?