Hello Nicola, welcome to the forum!
I believe 28L is a little over 7 gallons.
Have you done any water changes through this cycle? How have the fish been acting?
If ammonia is .6 ppm, I would do a water change to put ammonia under .25 ppm.
Its hard to say where you are in the cycle. My guess would be that you are still in stage one. Usually, you start getting readings for ammonia, then nitrites, then nitrates. In my tanks, ammonia is the first to go to zero, but will still have readings for nitrite. You still have no reading for nitrates. Nitrates are needed to eat up ammonia and nitrites, that is why I am guessing you are still in stage one.