If water gets between the upper and lower chambers of your BOROUX water filter, it means that the lower chamber is too full. This happens when the upper chamber is refilled whilst the lower chamber is not yet empty. In this case, the water simply continues to be filtered, causing the lower chamber to overflow. The water then leaks out between the two chambers.
It’s best to fill the top tank completely the first time you fill it. Let this water run through. If you notice that the lower tank is almost empty (as indicated by the sight glass tap or the water pressure), then fill the upper tank right up to the top again. You can do this because the upper tank is slightly smaller than the lower tank and, in addition, the upper tank contains the filters, which take up space. The lower tank therefore has excess capacity compared to the upper tank. By doing it this way, you prevent water from leaking between the two tanks.