if it's an Aquaclear filter - grab a flashlight and look into the filter. The filter media has a habit of becoming buoyant and rising up inside the filter body, raising the water level and creating a trickle leak down the back of the filter. My solution for this, remove the annoying useless basket, and drop the media directly into the filter body. That way you gain a bit of room, and instead of the buoyant media, which in mine is the carbon, lifting everything, it only lifts itself and the ceramic media leaving ample space between it and the foam for the incoming waterflow.