Hello again. It's been a month since the previous post, and I take it the problems were unusual enough that nobody could give any firm advice. So I thought it might be useful to say how things have gone, in case anyone ever sees similar problems and comes across this.
As you might expect, the last rainbowfish did not survive. I was later told the warped spine might have been from a bacterial infection, probably brought on by other stresses. Inspection of the body showed what would prove a clue to the underlying cause, namely tiny lesions on the head, almost as if there were extra nostrils.
After this a ram cichlid fell ill. The main symptom was spending all his time hovering in a single spot near the top, which in retrospect some of the rainbows had done, without much interest in food. He also started turning dark, which had also happened to some of the tetras shortly before they died.
As luck would have it a dealer happened to mention the protozoan Hexamita to me, and looking it up I found these three symptoms are all typical – hovering near top or bottom, darkening, and head holes. So based on this diagnosis I tried moving the cichlid to the spare tank and treating that with metronidazole (Metro+). For the other fish I spent a week giving food soaked with Epsom salt, which they did not seem to mind.
This has given very good results so far. After a week the cichlid improved and now seems entirely recovered, and nobody else has fallen ill since; in fact the remaining tetras have greatly improved in activity. So it seems like this was the underlying problem there and a good way to approach it.
Regarding the Camallanus worms and snails it is difficult to say for sure. I'm not sure how to detect them before death. Since fish were ultimately exposed to snails in both tanks and now seem generally healthy, though, it seems unlikely they can be carriers, at least after a few weeks.
If anyone has any further comments I would still be interested. Otherwise, thanks to anyone who tried to figure out what was going wrong here, and I hope this can be helpful if anyone ever runs into similar problems.
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