What a difference a day makes. I drilled holes, plumbed the growbed, moved the 520 lbs of rock, and added water. This video shows the siphon working while I was adding water to the growbed:
I was all set to call it a day. Alas, someone (aka the imp) decided the fish tank must necessarily host fish. They borrowed the camera and got this shot of the poor unfortunate en route to its new home:
Fish irrevocably consigned to the brand-new (and murky) tank, I immediately added air and conditioned the water with Amquel water conditioner. I have no idea how much temperature or pH shock the poor thing endured - not to mention being terrified. The three other goldfish are still in their tank, so at least we're only doing search and rescue for the one fish.
I had to get a new water pump and fiddle (a lot) with plumbing, but I finally got the autosiphon working. In a day or two the water should clear up enough to see the fish, whatever its state. Here's a video showing the bit parts. Sorry about the last couple of minutes - you get my audio but probably can't see the lovely laminar flow coming out of the drain tube against the black of the tank.
Phew! Now to get some rest...