In that time period I bought (and didn't adequately quarantine) quite a few fish. The fighter is an unlikely culprit - not only did we buy him too recently, he also shows no signs of infection. The pygmy corys are also unlikely, since they have never been in the main tank. On the other hand, the plant tank has a population of copepods, so I would only have to introduce the copepods, not actually any fish. It's reasonable that Camallanus was introduced with infected copepods that came with the Java moss. The timeline is reasonable - about 55 days. I also transferred a couple larger corys from the plant tank to the main tank, and an Otocinclus. They are also potential sources of infection.
There are, of course, other possibilities. I bought three batches of neon tetras this Fall, and had remarkably high mortality. I also bought some ghost shrimp. The ghost shrimp themselves are unlikely vectors - I have not read about Camallanus infecting shrimp, although there's a slight chance that they were using them as secondary hosts. The neons, on the other hand, are another story.
I bought my first batch of neons back in early September. Four of the five died within two days, and that was followed by a wave of mortality: a fighter, two Angels, two platies and a couple Macropodus. It seems pretty obvious that they weren't the ""cleanest" of fish. The extra burden of Camallanus infection could be blamed for the death of the neons (if you're already feeding parasitic worms, you have fewer resources with which to handle stress), it seems unlikely that transmission of the Camallanus larvae could have resulted in such rapid mortality among the other fish.
While I initially blamed the pet store, the deaths of the other fish led me to wonder whether something had gone wrong with my water, and that the timing might have been coincidental. Anyway, the sole survivor remained in the tank, seemingly healthy, but with a shrunken abdomen (which was always a cause for concern). He died a couple months later, after I bought some more neons to keep him company.
I'm most inclined to blame the neons (or more specifically, the one neon). And while I could do a lot to improve my quarantine procedures, segregating new fish for 2 to 4 months just isn't something I can do at present. While I hate the idea of medicating new fish as a precautionary measure, I can see why people would do it.
- Levsen, A. (2001). Transmission Ecology and Larval Behaviour of Camallanus cotti (Nematoda, Camallanidae) Under Aquarium Conditions . Aquarium Sciences and Conservation, 3(4), 315-325. DOI: 10.1023/A:1013137801600
- Levsen, A., Berland, B. (2002). The development and morphogenesis of Camallanus cotti Fujita, 1927 (Nematoda: Camallanidae), with notes on its phylogeny and definitive host range. Systematic Parasitology, 53(1), 29-37. DOI: 10.1023/A:1019955917509