I have a professor who is an enviromental engineer with a strong speciality in aquatic systems. I talked to him about this today and it's something right up his area of expertise.
He said (from what I can remember amongst all the scientific stuff he was saying) most bleach you buy has about 2-4% chlorine, which is what is really great at killing stuff. He said 1 ml of bleech per liter of water should be more than enough to keep everything dead inside your system. However that will cause a slightly acidic PH, which will have an impact on the metals, so you'd want to add a little bit of a base (don't remember which he said) to make the PH neutral.
He also said general purpose cleaners/disinfectants that don't have amonia might be an alternative.
Also, distilled water would work good, but only for a little while. Salt water would also work, but you'd have to watch and make sure it doesn't precipate inside the system (especially if your using a block like the cascade, with lots of narrow parts).
As for the antifreeze thing, the reason nothing grows in your car is that those temps hit around 300 degrees, which is enough to kill them. There are lots of 'little critters' that love to 'chow down' on that stuff, which has caused some huge enviromental problems at airports and other places. Many of those organisms also grow the best at 35 degrees C.
Just some info that might help some of you. If anyone has any other questions I'd be more than willing to ask him. He really knows about little forms of life that live in water.
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