I wouldn't say harmful, but I have seen some people put some different colored blinking LEDs in their tanks and the fish looked very stressed which makes them more vulnerable the disease. Their are a lot of LED fixtures on the market made specifically for saltwater and freshwater aquariums.
I have on a timer a cheap little LED light strip that turns on in the morning before my main lights and after the main lights go off. Even though the LED strip isn't intensely bright, it does bring out some interesting colors from my tank, fish, and the coolest part is I get a nice shimmer effect kinda like you would see using metal halides.