FYI, I'm betting the fish just died, because I got an ammonia spike (just .25) a few days ago. The fish wasn't sick as far as I knew, I had qt'd it for a month before putting it in main and it was eating well. It became part of the "pack" at feeding time. But it just disappeared.
Anyway, the spike makes me think that it died because I don't know what else would have caused it. Nitrates are still at 10. I did a water change and re-tested, it seems okay now. I HATE that! No explanation, just gone.
I thought I had a mantis a long time ago, and I know I have a eunicid... mystery.