A 50/50 bulb is half daylight and half actinic blue. Your normal bulb that comes with a full tank setup is usually more of the yellow/red spectrum(daylight). 50/50 has half that and half blue/violet spectrum.
The above post is correct. With 50/50s, you don't need to look at the purple actinic glow. You can combine the two effects but you still must use the same # of bulbs. The 50/50s have a broader blue peak than either daylight or actinic tubes.
