Ok, I'm in the US, what's the equivalent of 21 L? Wait, that's less than 6 gallons, right? You have way too many goldfish in there for 6 gallons. I've just learned that one fantail needs 20 gallons, then 10 gallons for every fantail after that.
First thing, get yourself a test kit. You and only you are responsible for the well-being of your fish. In the case of an emergency, like now, you need to be able to take readings. You should also be keeping a log of your readings for comparison purposes if a fish gets ill.
It's most likely your fish is suffering from ammonia poisoning. How often do you do water changes?