No expert but when mine did this it was too much ammonia in the water. I made the mistake of changing all the water but by doing so I was destroying the good bacteria. I got some stuff to put in the,water and some high quality food so fish produces less waste. My local independent pet/fish shop were very helpful and can test a sample of water.
It's unlikely to be the water change that caused the problem, but rather a build up of ammonia, nitrIte, or nitrAte. Ammonia and nitrIte are both bad and the level should be zero all the time. NitrAte is the end process of the nitrogen cycle and is only harmful in large amounts. I'm probably making it sound more complicated than it is, so this might be more helpful.
20 litres is in no way large enough for a goldfish. They get big. They shit a lot. They need lots of space and a massive filter. He's probably become stunted which won't have helped. Not blaming you for all this, there's no consumer protection against shitty, unsuitable aquaria being peddled by money-making companies who see little swimming dollar signs instead of fish. This will give you some idea of the size goldfish reach and how much space they need. I suggest a stiff drink while you read it!
So really, to help him you need to get him into a bigger tank (or a pond if he's a regular rather than fancy goldfish). Sorry I can't be more help
I know he'll need a bigger tank soon (he's grown quite quickly) but just looked at 140l tanks, and they don't seem that much bigger than the one we have, so I might be wrong about the 20l, and therefore I haven't been putting enough of the stuff you put in to make the water safe!
I didn't even understand any of that ammonia stuff in your link.
Ah no, there's not much to it. Basically the fish poo ammonia. Ammonia is bad and will kill the fish, but if you have a filter then the good bacteria will turn the ammonia into nitrIte (which is also bad) and then nitrAte (which is only bad in very high levels). You change the water to reduce nitrAte. The ammonia and nitrIte should be taken care of by the filter.
How long is your tank? Is it 20 gallons maybe? That's only slightly smaller than 140 litres.
If the fish is sick make sure you do regular, massive water changes. Ignore the poster who said they changed too much water and lost the good bacteria, this isn't true. Beneficial bacteria don't live in the water column, they live on the hard surfaces inside the tank and in the filter. Changing water won't hurt anything so long as you dechlorinate it properly. Easter changes are essential.
Get a reliable liquid test kit and test daily for ammonia and nitrite. Keep them under 0.5ppm and do large water changes to lower it. Read up on "fish-in cycling a tank" and your fish will be safe.