You should first check that the filter is positioned correctly in the top tank and pushed all the way down the hole so it sits on the ledge. It can happen that the filter is not level and the water leaks on one side. A good option is to remove the top tank while the filter is in place and grab the bottom of the filter while using your thumbs to push against the bottom of the tank. This enables you to manoeuvre the filter into place and to make sure it is level. Once you have done this you can fill the top tank with a litre of water to test that the leak has stopped. Another option is to remove the silicone seal that sits around the top of the filter just below the cap and to then replace it. This may help to remove any small dents that are allowing the water to leak.