Is UV Lights harmful to marine fishes?

Answer 1

To a degree, yes. They're harmful to everything living if in high enough intensity and for a long enough duration. It also depends on which type of UV as to how harmful the radiation is. UVA is essentially heat. UVB is required by some animals, but is also responsible for skin cancer. UVC is what's used in UV sterilizers to control algae spores and bacteria in tanks and ponds.

The danger of UV is not that it kills, but that it causes mutations to occur in the DNA. This may keep an organism from reproducing, or it may prevent the production of some enzymes or proteins that an organism needs to survive, thereby killing it indirectly. Not every cell is affected equally, so organisms that are only a single cell , if a mutation occurs, will be more likely to die than a multicellular organism. If only a few cells are affected, the rest of the cells may be able to compensate. But there's a danger of the mutated cells forming a tumor or becoming cancerous.

UV also has some beneficial effects, such as vitamin D production needed for calcium absorption.

Answer 2

Well the answer is yes and no. While given a high enough intensity and long enough duration sure UV would be harmful to marine fish. However I doubt you could afford the light bulbs or the electric bill to attain that type of intensity. Copperead gave a good description of UV and how it mutates cells and kills them. However uv light on that scale is not very attainable in an aquarium. A good way to think about it is how the sun reacts with people and the ocean. People with to much sun exposure(the UV light from it) can get skin cancer. Also the sun produces a lot of UV rays. You don't see the ocean dying do you? It's all about how much of an intensity and how long it's exposed to it.