Make sure that leaking is not just water coming out of the gaps on the
top of the filter. When I filled my fluval msf 404 for the first time I
pored some water on the top of the filter accidentally and I in
particular use that filter on it's side so that it will work with a
tank that is low to the floor of the room. When I turned the newly
filled filter onto it's side for use a lot of water came out from what
I discovered to be gaps in the top and when I left it alone for a few
days (I would have noticed much earlier if I had looked harder) the
carpet beneath the filter was totally dry and I now know why the water
pored out of what I now know to be gaps in the filter top. HTH, later!