Driftwood and catappa (Indian almond) leaves produce tannin, which slightly stains aquarium water, softens it, and lowers pH. I wonder if other decaying plants do the same thing. The streams and rivers where wild angelfish live have lots of aquatic vegetation and soft water.
According to Mr Google, tannin got its name, because it was used for tanning leather. The primary domestic source was oak bark. In leather curing strength, the stuff is toxic.
I don't know... but that's interesting.
You know there's a lot of decaying vegetation in the fishs' natural habitats...