It's been a busy week since my last post, but I have some good news. One of the fish wasn't really dead, only mostly dead. I discovered this while examining him looking for some indication of the problem. He flapped at me quite sternly, causing me to jump and eek. Suitably embarrased and encouraged by my son's laughter I took some pictures of the ailing fish.
Two dozen fishkeeping websites later, I believe my fish had a bacterial infection causing fin rot and bloating. This was most likely caused by trasport shock in moving to the aquaponics environment. This was further exacerbated by my attempts to solve the wrong problem.
I added 450 grams of Morton Kosher Salt to the system to bring the salinity up to ~2ppt. The fish have significantly improved, though I euthanized the mostly dead fish. Last night and this morning I saw three fish alive and swimming in the system, and they ate a few pellets. This is a really really good thing, since I haven't seen any of the outside fish eat _at all_ since moving them out.
On the business side of things I am expanding my search beyond lettuce dwc production. The idea of being tied so strongly to a single commodity product is unpleasant. I think some variety would reduce the total risk.
Hoping for continued improvement,