Hi Aces.. thanks for getting the photo posted.
Inka4040, 247plants, DeeSeven, and I spoke with Aces in chat a little earlier about his goldfish, so I'll add the details about this that we already have.
This is a common goldfish that measures 4 inches long at the approximate age of 2 years. Tank size is 10 gallons. Aces described a mass that is growing along the right side as being under the skin.. like it's a big bubble inside.. and the scales in that area are being pushed outward. It's also red and looks to Aces like it's filling with blood.
At this point, we talked about growth and stunting... and the possibility of upgrading to a larger tank size. Aces told us he can't do that.
So this is where we are -- At 2 years of age, the goldfish should have grown already to around 8-10 inches long. As a fish grows in a tank that's too small, it naturally produces hormones that keep it from getting bigger. Unfortunately, this only does that to the body.. the eyes and the internal organs continue to grow normally, despite being compacted by the restricted size of the abdominal cavity.
The mass causing this bulge could be a few things. Given the circumstances from stunting, the internal organs are likely being pushed outward and have reached a point now that it's caused this bulging on one side. There's also a possibility of a tumor inside. This could also be swelling due to an internal bacterial infection.
Aces... now that you've figured out how to post photos, would you take a few more pics for us to see? I know it's hard to get goldfish to sit still long enough to take pictures, but this would help us see better. If possible, try to get a photo of him from a side view. Another one looking down from above would also show us more about the bulging.
Other than the stunting and this mass, I don't see any other signs of illness in this photo. There's a nice wall of bubbles behind him that shows the tank is well oxygenated. You've clearly done a good job maintaining this small tank as well as you could. Over the last two years, you've managed to avoid a whole range of health problems that typically happen when a fish is overcrowded this badly.
Even so, your 10 gallon tank is not going to cut it. He has overgrown inside and is not going to survive where he is anymore. The stunting is so severe that it may be too late to save him at this stage.
Your only real chance to do anything for him is to get him into a bigger setup, keep the water quality pristine, and feed him a very high quality diet. You may not be able to get him into a brand new aquarium, but you could surely find a 20-30 gallon used tank on Craigslist for very little money. You could also get a huge Rubbermaid storage tub that holds 30+ gallons of water and set that up on the floor somewhere with an air pump and filter. You really need to explore these other options.
There's still no way to be sure this will save him, but if he gets into a larger environment and stops producing growth inhibiting hormones, over a long period of time, he might be able to grow enough to relieve the internal compaction. Only time and adequate housing for him will tell us that.