Hello Again! (just replied to your other thread)

It's probably "Fin Rot." Although, I think he is a Crowntail betta, if it's getting worse I would head to PetCo and get some Maracyn, that seems to work great for everything! And they don't have it at the place you got Tiny Tim... Good luck, and let me know if you need more assistance.

EDIT: As a caution, I would not suggest using Melafix, It's been known to harm some betta's when not diluted. The reccomended dosage on the bottle is WAY to high for a betta, and most times could kill them. And it's a PAIN to dilute correctly!