You really don't need the Maracyn2. Improving the water conditions will help alot. Since its a small tank just add some salt. How often and how much water do you change? Do you use a water conditioner, if you have "city" water? What is the water temperature? Usually, finrot is from stress from not so ideal water parameters. Sometimes it is bacterial. Something doesn't sound right though. Sometimes fish won't eat when you first get them, but are normaly "pigs" afterward, especially bettas.

I would personaly use PrimaFix and Melafix I think you will find these easer to use and they are very reliable.

Fin Rot is a bacterial problem usually caused by bad house keeping.
Do you do a regular 30% water change and how often. Do you do any water treatments before you add the water to his tank.

Here is a link to my Betta Tank

[link=Betta 11Gal Desktop & Placidity 5ft Community Tank Photos]http://photobucket.com/albums/b209/keithgh/Betta%20desktop%20tank/[/link]