Don't sweat the oak stumps if you have a lot of water to dilute the tannins. I have never paid attention to the species that branches came from, but I don't overload my ponds with brush piles all at once. Of course, it can't hurt to avoid oak if that is an option. My now extirpated beavers did not get the memo and dropped trees in the pond indiscriminately. Fortunately, my fish also did not get the memo and did not seem any worse for wear:)