Like he said, distilled water and a bit of anti corresive additive is what I use just to be safe, besides it keeping stuff from growing in the loop over time.
As for "T" line or rez, I like a rez better just because it's easier, I've run both types on my loops and there wasn't a different in performance that was measurable (even at max OC's)
I'd order a bit more tubing though, it's better to be safe than sorry, with one bad cut with 6', you'll be screwed and have to order more.......................
As for draining and flushing, it's pretty safe to go up to a year with a system that doesn't have mixed metals in the loop, but I'm into max performance and flush mine every few months or so.
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