Green Tea is more natural than black tea because it doesn't contain caffein, never fermented, and won't stain the water as dark. There are hundred of green tea varieties and some are claimed to have certain medicinal effect. Many are packaged in tea bags and so they can be adminstered with the right dosage with no mess. They will lower the pH to around 4 and is close to many black water condition. If I were to keep black water fish, I will certaubkt explore tea as a simpler alternative to peat or RO filtration.