What kind of test kit are you using?
Are you sure the tank is cycled? You post it is from Christmas but you posted late January about starting up a 20L. When did you add the fish? Temp? What is your water change schedule? I suspect you have an ammonia spike and something is amiss with your testing.
I have a fishless cycle going on in a tank now that is is currently on day 31 and I still cannot add fish and I too used filter media from an established tank. amm 0, nitrite off scale, nitrate off scale. I should have the nitrite fall off any day now and then I do a water change and am done but it is not something that can be rushed. PH and temperature affect cycle times too.