Rooibos tea adds tannins to the water, which colour it brownish and help the fish's slime coat and immune system. They also make the water a bit more acidic, which bettas like. You can also use Indian Almond Leaves (IAL) or dried oak leaves. You can buy IAL on ebay.
I found plain organic rooibos in my local chain grocery store, they were the store brand. Soak the tea bag in the water until it turns a medium brown colour, then remove it.
Note: If you add leaves to the water, make sure you add them to fresh water when doing a water change.
decaf green tea, Ive heard is also ok. but dont just used any tea in your fish's tank, you can and will kill your fish. especially since bergamot is a citrus fruit, it will probably make dramatic ph differences in the water, if caffeine didnt already kill your fish, 90% the ool will