Fish Tank Litres Test Why Does My Fish Tank Smell?

Why does my fish tank smell? - fish tank litres test

A few days ago I bought a tank of 25 liters. I have a filter and an air pump in and they both seem to work well. I put two goldfish yeaterday morning and seems to be a solution in the wells are examined and good to eat. Today, however, I noticed a smell of urine in the reservoir. There were live plants, but they were not so good and so I took it out and replaced by articficial. I also have a test for ammonia, which showed that the levels of ammonia are very weak. I have a water change of 20% in the morning, but the water still smells like always. What is it and what can I do?