Goldfish growth issue

[tarmac]

Hi Folks, One of my original goldfish had developed a large growth which he has had for some time, however recently the growth has enlarged and looks to have burst? The worry is that the growth looks like it is now rotting and has gone a white-ish colour. The fish still appears happy enough and is eating okay.
Can anyone kindly offer any advice? Is there anything I can do for him?
Thanks in advance. (Sorry about the photo, the reflection on the water makes it hard to capture a clear photo)

 

[Dawgfish]

Could be bacterial infection or a tumor?