Series or parallel wiring is a consideration to be made once a specific type of lighting is decided on, ballasts, if any, are chosen for the system, how many bulbs, etc etc etc. It is not ever a generalization, exepting to say maybe that Parallel wiring is more common in properly designed systems, as Series wiring is not usually desirable for multiple load sources that are codependant, especially lights. Do you remember the old x-mas tree lights where if one bulb died, the whole strand would go out....? Besides, these generalizations can be misleading, like a 2 bulb ballast that has only 3 output wires... each bulb may get its own hot, but they share a neutral. It's parallel, but to a newbie it may be confusing due to not truly grasping the concept. etc.
Mercury vapor, halogen, and high pressure sodium are all tempting lighting choices for the planted tank enthusiast due to high availability and low cost.
However, these types of light generate mostly yellows and also get way hotter than other types, except maybe metal halide. But, at least the metal halide provide a much fuller spectrum of light and run less kilowatts on your power bill so it's a fair trade off. Using Halogen, HP Sod., or Merc. vapor will cook plants before it grows them in most cases.
I am an electrician among other things. and have done more than enough research and work in the lighting field. If you have any specific questions, I'll be glad to help.
