Hi Dave - if you read the thread in the link I posted earlier you can read about what type of leaves I recommend, how to prepare them, why we boil them and also the answer to your question - in short, it's safe for any fish that will also tolerate driftwood in a tank.
Oak leaves release tannins, just like driftwood. They also leach humic acids - the result of which is a minor increase in acidity ie a slight pH drop can occur, depending very much on the KH of the water - lower KH will see more of an effect on pH whereas with a higher KH where the water is more able to absorb an acid without becoming acidic, the difference would be insignificant. It's a gradual release too, so nothing happens too quickly.
We did chat about it a bit on the CF where I provided that link, there might be more info there which I forgot to mention.
Blair.