I agree that varios woods/leaves introduced into a pond can release tannins. It seems that high levels of tannins can be detremental to the fish population. I belive that by ensuring a high alkalinity and hardness of the water (through liming or good alkaline pond substrate) you will buffer the effects of the tannins. However, with a heavy introduction of tannin producers in a pond with low alkalinity/hardness, the tannins can take over. My advise would be to initially boost the alkalinity to good levels before introducing potentially tannin producing vegitation. They should be gradually introduced in order to make adjustmants if things start to get out of hand.
An interesting aside...I read in a few articles that tannins may play a role in protecting fish from some diseases and parasites... but keep in mind... you cant believe everything you read on the internet.
Sarc
