Tannins can be from-naturally dried and fallen from the tree Oak leaf, Indian almond leaf -use one of these tannin sources and add to the pre-mixed treatment water to steep-the longer they steep the more tannins will be released the darker the water the more the Betta will like it
Blackwater extract and even decaf green tea-these will also provide tannins-however, I do not know if they also contain the antibacterial/fungal properties that oak leaf and IAL have-but they will tan the water and this can sometimes be helpful for a stressed fish.