Peter May says…
Laburnum and yew have to be at the top of the danger list of trees, since every part, including the pollen, is poisonous. Alder buckthorn also has poisonous leaves, bark and berries. Any tree that sheds foliage or berries into the pond is liable to overload the biological filter system, but the leaves of willow contain salicylic acid – the basic ingredient of aspirin, which is highly poisonous to fish. Oak leaves contain tannin, which in large volume can also be poisonous. Smaller trees and large shrubs include the guelder rose (Viburnum opulus) that is poisonous in all its parts, and to a lesser extent the rhododendrons, oleanders, wood spurges and daphnes. Holly berries can be poisonous in quantity. I have always been told to avoid planting elder near ponds, but cannot find any written evidence for avoiding it. Dodgy climbers include the wisteria, the solanums (avoid all members of the potato and nightshade family) and honeysuckle and sweet peas, all of which have poisonous berries and seeds. If you let ivies flower and seed, these too can be poisonous in quantity. Large perennials often found in boggy regions may cause problems and include the giant hogweed, foxgloves and larkspur. Even bluebells and hellebores are best avoided.