Poor Goldie's Fins are almost completely gone!

 

I was shocked over the last few days that Goldie's tail and other fins are now almost completely gone. I am wondering if its fin rot (which I am treating the aquarium for, or maybe he's getting nipped at by Whitie, who is 3 times larger than Goldie. I have been keeping fish for over 30 years and have never seen this. His tail is now completely gone. From what I can tell he can still swim and eats but has become quite skittish.

Anyone that has ideas please let me know!