[nelly]

Has the tank been off at all lately? The O-Rings on the fuel connectors are very easily damaged.
The other "seal" on the tank is the O-Ring on the pump housing. These usually either seal or leak. Once fitted they remain leak proof, but if it's just had a filter change then that could be the culprit.
Only other leak could from the breather tube has become detached from the T-piece on the base of the pump.