Found the problem. There is a crack about 9cm on the top cover, where the O-ring is. The crack must be a manufacture error cause theres no way I could have done that. When you place the O-ring around the socket isn't there a gape in the cover, well its in there...
I recently bought a Fluval 405, no more then one month. From day one it leaked when I tilted it to drain the water. It didn't really bother me so didn't give it any attention. Today as always I unplugged the filter to feed my fish. After about a minute I saw water underneath. Water was leaking...