Possible brain tumor

Hello all, I hope everyone is doing good. I have a concern over my one peacock cichlid. I have had him for two or three years, raised him since he was young. As the years have went by, his bumb on his head has gotten bigger and is protruding more from his body. The bump also looks like a bruise...theres a little color discoloration. I thought when I first noticed the bump that this was just a deformity due to high nitrates. I had several cichlids and other fish in a 46g for some time then upgraded to a 75g with less fish. I think this bump is getting larger. With him being a cichlid, he is a fin nipper and likes to chase my other smaller peacock around the tank regardless of all the sight breaks I have. He is eating fine, swimming fine, normal behavior, doesn't look or act like hes in pain. Maybe he built up a pain tolerance to the bump....i'm not sure. I did some research and came up that the bump could be a tumor, bacterial infection, disease, jack dempsy bump or just a natural deformity. The bump is only on one side of his head, above his eye. Any thoughts? When they were in the 46g, ammonia was 0, nitrite was 0 and nitrates were 40. In the 75g, ammonia is 0, nitrite is 0 and nitrates are 10.