Well, I think we can give you come answers. 40 liters (approximately 10 US gallons) is much to small for that type of goldfish. The lump you are seeing may be the result of stunting, especially if the fish is that old.
Unfortunately, Local fish stores generally give people the wrong information. What you described seems to be a Common Goldfish, which need a lot more swimming space, as they usually get massive. Under normal circumstances they would need 75 gallons (something like 280 liters I think) minimum, but usually do better in ponds.
The other thing that may be an issue is the water. Goldfish are very, very messy poop machines.
In a tank that size, with that fish, it would require 50% water changes almost daily to keep the water healthy for a fish, two or three 50% changes a week at the bare minimum to remove the built up Nitrates and other toxins in the water. Changing the water is the only way to remove these toxins, since a fish tank is a closed system.
If you can, pick up an API Freshwater Master Test Kit and test your water parameters. Most local fish stores carry them, or you can get them cheaper online. They cost about 30 dollars in stores, but last a long time.
Testing for:
Ammonia
Nitrite
Nitrate
pH
Will give us a better understanding of what it is we are dealing with in your tank.
Sorry to be the bearer of bad news. I hope that information helps! Clean water always helps, but overcoming the stunting will be next to impossible. I know jlk will have more precise answers though!