Its quite unusual for liquid cooling systems to freeze up (they are connected to a heat source), unless your radiator is outdoors, but you could add antifreeze to water to keep that from happening. The quantity seems to vary from 10% to 75% according to this thread. It has the bonus of being anti corrosion and possibly being a biocide. Antifreeze is nasty, so, you'd want a sealed setup rather than the bucket.
The only reasonable alternative i have heard of is mineral oil, but thats usually used for immersion cooling, due to its viscosity