My first fh i ever had the same problem too when he grew up to be 12inch. I tried everything and it didn't work. Then a local friend whose been in fish hobby for years said that it could be my water. My water could have metals, like zinc, copper, lead and he said that when fish been raised in water like this. When they get big they will start to develop tumors like on there body. I told him i use RO water and add conditioners, but he says thats not enough to remove the metals. He told me i did everything right except aging the water. He said aging the water will remove more metals, but aged the water for at least 1 week. I did what he said and months by months the tumor on my fh started decreasing. 11 months later it disappear, but it does having scarring or shades of his tumor where it use to be at. This is how i got rid of a tumor on my flowerhorn.