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.