Help! Haze won’t go away

Help! Last week I did a major water change in my aquarium (approx. 50% of the total 120 liters), also replacing the filter contents which were old. I also added a new fish: a white bristle nose. A day later the water suddenly became cloudy. First white hazy, but now it is more of a green haze. I changed some water again, but the haze remains. In the past week I tested the water several times with the JBL Pro Aquatest (case). Both NH4 and NO2 and NO3 came out well in the test. The aquarium is on for about 7 hours a day during the week, and 10 hours a day in the weekend. What is this, and what can I do about it? Thanks so much in advance!