I'm not familiar with the tank but I would take the atomiser out and compare it with a new one to make sure it has no 'O' rings missing. If all intact, I would screw it back into the base tightly with your fingers. Also check there is a 'O' ring fitted on the chimney where it screws into the top of the atomiser. I take it this is the top fill one where you slide off the top cap to fill through a slot. On some of them, you unscrew the top cover from the atomiser to fill, and this can also loosen off the atomiser from the base.
I take it the leaking is coming from the air holes at the bottom. (Should of asked that first....lol)