I am no expert using ketapang, in fact, I havent not tried it before. But here's some links to useful information on it.

Add if you have some good information to share.

Understanding BLACK WATER

THE MULTIPLE BENEFITS OF THE DRY INDIAN ALMOND LEAVES

Ketapang Leaves or Huu Kwang Leaves or Indian Almond Leaves Explained

Ketapang leave in - Caring for your arowana