I have green water in my aquarium, and even water changes and changing the filter media only helps for a short time.

Green water means floating algae, which grow when they have no competition from other plants and, even more, when the excrement of the fish creates excessive nutrient pollution in the water. The filter will not be able to do much about this. Ultimately, there are only two possibilities:

1. Carry out significantly more water changes in order to remove the nutrients caused by fish excrement.

2. A JBL AquaCristal UVC clarifier kills off the floating algae.

A temporary measure is to switch off the light for 2-3 days in order to stop photosynthesis in the algae.