Garlic is a good natural way to eliminate parasites. Get some FRESH garlic cloves & crush it/chop it into teensy weensy pieces for your betta and feed it the whole bits. The bits of garlic will sink right away, just to let you know. Or, you can soak your betta's food in the juice of that same fresh garlic.
If you do a little google search about the health benefits of garlic, you'll see it's a natural antibiotic, antifungal, antiparasitic, and antiviral.
Here's a short but interesting article: