There are many uses when it comes to using dry Indian almond leaves.
Inducing Spawning
If you want to breed some fries out of your betta couple, ketapang are a great tool for inducing spawning.
Lowering pH and Ammonia
If pH and ammonia are too high in the water, it will do damage to the fish in the long run. These leaves are great for this purpose.
Building Bubble Nest
Male bettas need a safer place to store their eggs and dry Indian almond leaves are fit for the job.

Curing Diseases
Even though this is not a one fix for all, it will definitely help in curing diseases carried by the fish.
Killing Parasites and Bacteria
They are the agents of bringing the parasites and bacteria down.
Water Softener
They will soften the substance of the water.
Black Water Extract Effects
Some fish and reptiles need the black water extract. The leaves will just serve the purpose.
Curing Torn Fins and Tails
This may not always be the case. However, it still contributes to the job, somehow.
There is really no specific way of using ketapang leaves for your fish.
Basically, you just need to keep any of the leaves in the water and you will see the water turns yellowish.
When this happens, it means the properties of the leaf have already been released, and they are already working in the tank.
How much should you use then?
 
How much to use is a more personal choice with some wanting a darker and more in the tank with some just wanting a pinch - how your fish react is one of the best ways to measure.
If I discover my fish is not feeling well and its fins are torn, then I will crumble a larger portion of the leaf with my hand and dump it in the tank.
That way, the properties will release much faster than usual.
If you have a large tank, you can place one whole piece of leaf together with the fish, be it betta, tetras or even arowana.
Breed bettas, you will also need to place a whole piece of leaf on the surface of the water and let it float so that the male betta can make bubble nest when he is ready.