Will a black light create allege in your fish tank?

A black light configuration is completely safe for your fish. It's more recommended for aquascaped fully-planted aquariums, as it prevents algae blooms. If you do decide to use black light, make sure you have plenty of decorations that react to it, otherwise you won't have much use for it. I have a 45 gal. tank set up with a black light for night time, and a fluorescent for the day. It's cool to have less light coming from the tank, and when you stumble out to use the restroom in the middle of the night, and all you see is glowing fake anemones or plastic plants, its pretty awesome. =]

Good luck, and happy aquascaping.