I would guess your tank is leaking where the mounting studs are welded on?
To keep from overtightening these lame studs, mask off the top tube, spray the area where the tank will sit with plasti-coat, then mask the tunnel and spray a layer in it, too.
When remounted that tank will stay put without overtightening the nuts and also helps with vibes shaking the tank up.
As far as fixing the leak, can't help ya there. You could do the cream gas tank treatment but it costs more than a new tank, so...