Under water lights installed in a pond extends the enjoyment of the pond beyond the daylight hours. The pond takes on a completely different look at night with lights in it. At night, because there is no glare from sunlight on the pond surface, the koi become more visible than ever.
The only complaints we ever hear about pond lights is that I wish we had installed more lights when we built the pond. We usually recommend installing multiple lights in a pond, depending on the size of the pond.
Generally, the purpose of adding lights to a koi pond is to light up the koi, not just the pond. When you only add one light to a pond you often times end up with one stream of light shining across the pond while much of the pond is still in the shadows. With several lights coming from different angles, the whole pond lights up and thus the koi become very visible.