Do Pond Lights Bother Fish?
The short answer is, most likely, no. But let’s go into more detail about how pond lights may be harmful if overused.
Of course, fish were out there, in Nature, long before humans started building ponds. There is light in nature, obviously. Fish are used to it unless they’re living at the bottom of the ocean or something. Fish are used to the sun shining on them during daylight hours, so the question is really whether extra hours provided by underwater pond lights will affect them.
One way to test, is to put the light up to you and look at it. Is the light so bright that it bothers your eyes? Then it could bother the fishes’ eyes as well. However, keep in mind that the light brightness will be diluted by the water. Most lights you find for sale are going to be safe for them, especially if you provide places in the pond where they can hide from the light if it gets too bright for them. Also, lights make your pond more visible to predators, so keep that in mind, too.
Most pond lights sold these days are LED, which run much cooler than traditional halogen lights. Halogen lights, though, might heat up your pond water and could be disturbing to fish
Whatever you do, the best idea is probably to put your lights on a timer, so that they run a few hours at night. That way you know that your fish can have a dark night, too, undisturbed. The problem with running them on a photocell is that then your fish are subject to light 24 hours a day, and this is not a natural environment for them.
Solar lights for fish ponds are not nearly as common as those sold to run on the grid. If you do decide to go with solar, note that you will obviously need a battery to run the lights at night.
At Fountain Mountain we have dropped our pond lights for small ponds, as they can be found so cheaply on sites like Amazon (which has very, very inexpensive shipping costs to the company), and they weren't worth the hassle. Some bulbs inevitably burn out and can be upsetting to customers. All in all, small pond lights created a heck of a lot of warranty issues. We now only sell pond lights for large fountains installed on large ponds, e.g., at least a tenth of an acre.