The water cleared while I had a serious problem with cloudy water and algae. What I need to emphasize for those interested is something that even in communication with the Greek representative they made a mistake. In order to avoid a decrease in flow, the diameter of the filter hose must match the diameter according to the model chosen. Also, the size of the aquarium doesn't matter as much as the flow needed to clean the water or kill bacteria. This is written on the box. If your filter has a flow rate of, let's say, 1000 liters per hour, then you should get the UV 800, which can clean the cloudiness in a water flow of up to 1200 liters per hour. In other words, your filter should have a lower flow rate than what the UV lamp can clean.