Ponds, Trees And
Most people think that ponds need shade to stop them from going green. This is true but they also need sunshine to make the plants grow, this balance might seem impossible to achieve but really is quite simple. All floating plants, whether they are lilies, duckweed, fairy moss, or hyacinths will provide a degree of shade, as will marginal plants around the edges. For these plants to grow healthily they need plenty of sun; Indeed the most common cause of lilies not Flowering is lack of sunlight.
The first year that a pond is installed it may need artificially shading for the protection of the fish that can get sunburn, but after that sufficient surface area should be naturally shaded by the rapid growth of the plants. Never plant trees close to a pond, or position a pool close to overhanging trees for shade. There are a couple of simple and important reasons for this, firstly if it is a deciduous tree such as a willow, which incidentally loves being by water and may seem a natural choice, it will have thick roots which may well damage a preformed or concrete pond and even split a liner.
Secondly trees will drop their leaves every year thus producing an annual problem that lasts for several weeks. The leaves falling onto the pond must be netted and removed regularly, or else they will rot and pollute the water. Netting the pond can be a partial but unsightly solution and is never totally effective. Conifer trees next to pools are just as much of a problem. The roots are fine and fibrous, but the needles drop constantly throughout the year and are difficult to remove as well.
Furthermore they are a lot more poisonous than most deciduous trees as they are acidic in nature. Even flowering shrubs nearby can be a nuisance as the leaves will blow in the Autumn and in the flowering season, blossom can become a headache for the pond-keeper blowing everywhere. If you really want to provide shade for your pond and it is a very formal square Koi pond with no, or few plants in it, why not build a roof over it and tile it making a feature out of it along the lines of a Japanese building. This will also keep the snow off it and help to make it a little warmer in winter that will benefit Koi.