As far as i remember, there is not an airtight fit between the filter and the cover without the snorkal insert. To remedy this i used a doughnut washer obtained from a plumbers merchant. This is about the size of a ring doughnut but made from foam. It is too thick as it is so you have to cut it in half ( so you end up with 2 ). I found this to be a good solution and cheap aswell. P.S. this was using the standard filter and a DNA one . So maybee tour foam filter has a better fit.