Stress crack most likely caused by the metal thinning from tank rust. Best resolve if you keep the frame is to grind it well, if it is not weak when probed with a screwdriver then drill stop the crack. Weld the crack up, you would want to consider beefing it up with a patch over it anyhow.
Otherwise cut out the weakened portion and weld in a goodly patch.
ALWAYS add a upper stress bar to a monocoque frame if it has weakened (or if you plan to do any riding over 40mph or want to race the bike). Either a welded in one or a bolt on, though I prefer the welded in one. Make sure the frame is true before you add on the stress bar, if the steering head and seat post is not in vertical alignment, correct that issue first.