I use native collected Oak leaf in my aquariums and for treatments
For oak leaves-you want to first make sure the collection location is pesticide free
Gather naturally dried and fallen from the tree leaves-they should be brown, tan, yellow, orange, red...etc.....they don't have to be crunchy dry but dry, no green in the leaf or stem
Inspect-you want as perfect looking leaf as possible, no hole, gulls, wounds, bug eggs etc...
Rinse before use only-you don't want to get them wet until you are ready to use them
Use as many as you want-the more you use the more tannins you will get and the darker the water will become
Change/replace them out weekly with a water change
Remember-that your replacement water also needs to have the tannins if you are changing out more than 50% of the water
Oak leaf can change the pH to a degree-usually not enough to cause any problems but why take the risk.....
Fresh carbon in the filter will remove the tannins and you may not get the nice amber colored water the Bettas loves with it use.