There are two things that are most likely the main causes of your goldfish not doing too good.
One - the tank at 20x10x12cm this is only 1 gallon, really is far too small. It really is. The water the goldfish is in is getting too toxic too quickly as there is not enough volume of water to dilute the amount of poop and ammonia the fish naturally releases into the water.
Two - there is no filter. This is essential for ALL aquariums. Why? Because filters will move the water around the tank constantly and more importantly it will help get rid of some of the toxic water that I mentioned earlier.
Iāll be honest here, I donāt think things are looking good for your goldfish UNLESS you can do a number of things.
Get the largest container you can find, a large plastic storage box that is clean will do fine.
Or if you can get a bigger tank, even a 10 gallon tank is 10 times better than the 1 gallon tank the goldfish is currently in, Iād advise, if possible at all, to get at the very least a 15 gallon tank / 60 litres but youād be better with a 25 or 30 US gallon / 100 litre tank to be honest.
If this not possible if you have not got the money or the means to do so, try your best to at least get a large plastic container and clean that out (clean with just water, no soap at all) and fill with water for goldfish.
Add a filter, ANY filter is better than nothing at this point in time.
Add tap safe dechlorinator. Even though you say the water is safe. Itās safe for humans, this does not necessarily mean itās safe for aquatic life. Dechlorinator helps to take out toxins and metals as well as chlorine from the water, all that helps to make the water safer for the goldfish. This can be bought cheaply for pretty much any pet store that sells fish things.
Third - Change the water regularly, at least 50%, the more, the better.
That will do for now, when youāve done all of the above, let us know then and with as much information as you can provide.
We then can help advise what to do afterwards as things will get a bit more complicated after that.