It sounds like your set up similarily to me. I've got a "bank" of needle valves contoling flow to the tanks upstairs, open flow to a reservoir in the basement "laboratory". Can you block flow to certain "circuits" and do the process of elimination thing...?
That vinyl tubing
may be the culprit. I kinda' think it may be in the RO unit though these things are relatively new to me

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I was goona' invite myself over for a look, and a beer, 'cause I was down that way to get stuff at Home Depot but Margo kinda' rolled her eyes so I passed. Now that would have been a night from hell for the guy in the plumbing department if two fish geeks showed up askin' questions that defy all the ethics of plumbing...
