I keep seeing oak leaves specificially recommended for shrimp tanks. People use the brown, dead ones and boil them first.
This is from a poster on applesnail.net, who breeds cherry shrimp and other inverts:
"Put in a handfull of dead brown oak leaves that have soaked for a few days to remove tanins. As they rot they remove nitrates & phosphates (just like a compost heap does) and the bacteria are the cherry shrimps' natural food. The snails don't mind at all.
Avoid any sort of fast growing plant with cherries as they out compete the bacteria for nutriients. Java Moss & Java Fern is OK in limited amounts, but if you have enough leaves, the java moss won't grow as the nitrates and phosphates are all aborbed by the leaves.
Plants I've found to avoid in a shrimp tank are Riccia, Hornwort, Duckweed, Frogbit, etc. These plants compete with the leaf bacteria. In reality, the best shrimp tank has no plants at all, just leaves."
Remember that Darwin was mostly right!