If water gets between the upper and lower chambers of your BOROUX water filter, it means the lower chamber is too full. This happens when the upper chamber is refilled while the lower chamber is not yet empty. In that case, water simply continues to flow through the filter, causing the lower tank to overflow. The water then leaks out between the two tanks.
It’s best to fill the upper tank completely the first time you refill it. Let this water run through. When you notice that the lower tank is almost empty (as indicated by the sight glass valve or by the water pressure), refill the upper tank completely. You can do this because the upper tank is slightly smaller than the lower tank, and the filters in the upper tank 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.