Hi,
How many fish did you have in the tank. 28 litres is not much and from what I have read (never had my own goldfish) under the coldwater section of this forum goldfish are pretty messy fish. It may be that the tank is too small for more than one fish.
Ammonia builds up very suddenly, if your fish are messy it may be that the water will need to be changed every day or two - about 30% at a time. Another thing is that when you rinse out your filter you need to rinse it in the water you have taken out of the tank and not under the tap as fresh tap water will kill off any beneficial bacteria held in the filter and it is that, when established, that helps keep the ammonia levels down.
For more advice from those more experienced in keeping goldfish you might be best to post under the coldwater section on here.
Good luck