Hi Steve,
Yes, I've wired in series and in parallel. In series as you said, wired inner rod of set one, outer pipe of set one to outer pipe of set two, inner rod of set two to inner rod of set three, etc. with six sets at 12-14 vdc would give 2-2.3 vdc per tube set. Yet, only one tube set is producing. Likewise any parallel/series wiring results in just one tube set producing weird. Either there's a shorted tube set or loose connection somewhere not sure will check. Parallel definitely all tube sets producing. In your example were you using tap water? Thanks,
Andy