Is the UV lamp useful for decorative freshwater (aquarium) fishes or may it be rather dangerous instead?
As far as I am aware the Sun causes fishes' coloration to saturate, and although the UV lamp only resembles sunlight, it is still supposed to provide such effect.
And on the other hand, the irradiation is potentially harmful to people during protracted exposure; can it also be as dangerous to the fish?
The answer to your question is yes and no.
If you are using UVA light (black light) then this is safe for your fish; most of the light sources used on fish tanks do emit these wavelengths to some degree, but the black light sources are coated with a material that filters out the other wavelengths from red to blue light so it radiates only near ultraviolet and infrared light (heat).
The dangerous types of ultraviolet light are UVB and UVC; UVB is what gives you a tan or a sunburn, while UVC is the most dangerous type of ultraviolet light - this is used to kill bacteria and algae in water and to sterilize surgical equipment, among lots of other uses.
UVC radiated from the Sun is stopped by the atmosphere so it does not reach the ground, but it can be measured in Antarctica due to the ozone hole there.
More information could be found in this Wikipedia article about ultraviolet light.
Near UV = UVA.
Middle UV = UVB.
Far UV = UVC.
Nothing can be compared to natural sunlight.
I recently moved my fish from artificial lighting to my balcony - open to air, sunlight and without a lid.
The difference was evident within two weeks. At first, my fish were scared of the open. Now, their colours are significantly more saturated. The fish have also become incredibly active (and hence hungry). They also know when to dive down into the water when crows arrive.
My nocturnal aquarium snails seem to be happy too.
