What is this growing on my fish?

My fish is growing this large reddish lump. He is eating and swimming fine. Doesn’t seem to be in distress. What could this be? Help please

 

Oh that's quite a growth! I really wouldn't know where to start with that other than the obvious keeping water quality optimal.

 

Tumor to me.

 

Tumor to me.

Do you know if there is any treatment for this

 

Sorry but there isn't. In some way Black Widow's seem to be prone to some sort of cancer.