???
They're your fish. What's your ammonia level?
Is the 28 liter where the goldfish is now or where it is going?
I made pretty much every possible mistake starting out, so I say this with deep empathy- there's a lot of important information you need to know.
28 liters is too small for a goldfish. Fancy goldfish need at least 30 gallons, not because of their size but because of their impressive ability to produce waste.
Cycling your tank takes however long it takes. You need to be adding an ammonia source to build up beneficial bacteria.
It sounds like you're using test strips, which, aside from not being entirely accurate, don't have ammonia tests. I highly recommend the API master test kit. Get the whole thing. It's less expensive than buying individual tests.
We all make mistakes. The important thing is to learn from them.