Doesn't algea require sunlight?
Yes, but unfortunately what grows in WC systems isn't always algae. It's more often bacteria, which will grow just about anywhere. But what they do need is a food source. If you start with distilled water, disinfected equipment (boil your tubes!), and keep the system sealed, the level of organic material in the system for them to feed on will be very small. The total mass of bacteria in the system will never exceed the mass of organic material you start.
The pictures you occasionally see at places like procooling, with systems clogged by bacteria growth are the result of either an unsealed system, or a system where the little critters are actually feeding on the admixes added to the water. Things like bleach and anti-freeze, at low enough concentrations, can act like fertilizer, which is why they should never be added to your system. There are some reports of anti-corrosives sparking bacteria growth as well, most likely in situations where they're not being used at the proper concentration.
I'm a firm believer in the concept that good-quality distilled water (the truly distilled lab-grade stuff, not Walmart brand
) is as effective an anti-bacterial/ anti-corrosive agent as any of the commonly used admixes. (the anti-corrosives work by binding up the ions in the water, start with fewer ions, and there's less to bind up)
A clean system should run for months/years with just pure water.