Think ROY G BIV
For PC's and VHO's what happens is the white powder coating on the inside of the bulb (phosphers ?) starts to thin down thus letting more of the red/orange side of the light spectrum through. Corals use the blue green side and the grass on your front lawn uses the red/orange. So when you are emitting that color spectrum, pretty much anything plantlike will grow in it. Halides lose their intensity and emit the same results (more red light)
YOU can't see it but the stuff in your tank sure does.
Your cyano can come from a number of resources, but since you know your bulbs are old, that would be a good place to start.