How would you run 5 independent loops off a single channel radiator?
In series, just like I used to do with 4x GPUs. Can easily dissipate 800W+ of heat on the standard 240 rad I was using back then.
We don't have 800W of heat, we have 5KW of heat - you simply cant get the flow rate required to do that in series before you get to the radiator due to the internal diameter of the tubing [and pathways in the heatsinks].
You also dont need "specialized" coolant for any sort of watercooling, distilled water changed regularly will work just as well, at $.99 per gallon at your local store if that much.
As usual, I recommend the safe options because lets be honest, what percentage of people are going to decide to spend a few hours to drain, flush, clean and refill their multiple C1s?
You will notice I stated a standard 240 rad... that 9x120 rad with push pull could certainly handle 5 of these in series without issue. Infact... I might just do it.
You can also use a power powerful pump as Mr Teal pointed out. Ive been in the WC scene for a LONG time, built over 100+ machines needing to dissipate 1000+w inside a normal ATX case, and those parts were much more heat sensitive in delta T than ASICs generally are.
By my rough napkin calculations, a radiator with reasonable airflow should be able to dissipate a maximum (ideal) of 900w per 120MMx38MM Rad, giving you a theoretical maximum of 8100W dispensation for that radiator. Obviously I am intentionally ignoring things like airflow and water flow rate, but you get the idea.
problem with running in series is that you would likely see temps on each unit increase in succession, with the last unit(s) having the coolant entering at already-high temperatures and offering insufficient cooling. To avoid this, the flow rate (and thus static pressure from the pump) would probably be too much for the C1 waterblocks.