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my friend told me that black lights are good for the fish is that true? ???
Only to highlight colors on fish as far as I know.
I wouldn't hang a normal black light over a fish tank. I'd buy a moonlight specially designed for aquaria. I'm not sure if a regular blacklight's spectrum would be safe for freshwaster fish. (But I may be wrong.) Or, if you want a bright tank with blue hues, get an actinic bulb - but remember that with an actinic bulb over a freshwater tank, you'll get a lot of algae.
Moonlights in a freshwater tank allow you to see your fish after the regular tank lights are turned out. They neither help nor hinder your fish. In a saltwater tank I understand they are beneficial.
I'll do some checking about the black light as far as being harmful is concerned but I really don't think it woud be helpful at all.
my friend said that it makes ther colors brighter
Just makes them look brighter, doesn't change their color at all.
Black lights are completely safe. They are just standard fluorescents with a special coating that blocks out certain wavelengths of light. All fluorescents including black lights give off relatively harmless UV-C radiation so that shouldn't be a reason for concern. Black lights do make certain colors glow very bright which looks cool but not natural.