Yes, it was my own fault. In 20 years I have basically never quarantined new fish, and this is only the second time in 20 years that I have introduced a real problem. I would see ich now and then, but that is easy to deal with. But this protozoan arrived in some new Hemigrammus pulcher. They looked fine, active, nice colour. But within a week five of them disappeared literally overnight, I never did find them but they were small and in the 115g tank, and at the same time the other fish began dying.
The metronidazole added to the food appears to have worked. Fish deaths that had been 2, 3, even 5 a day ended last Thursday. The fish that are left are bright and active again, a few are even spawning, so I think this worked. Treatment will continue for a full 2 weeks. There is another stage of treatment that can be used if this were to be stubborn and resurface, namely adding Quinine to the tank water. But that has some risks for certain fish and plants, and hopefully won't be necessary.
One thing I have finally learned is to QT new fish. On and off for 20 years I have done this, but nothing ever occurred, and I thought that stressing the fish out twice instead of just once for no reason didn't make much sense. The first time I had a major problem was almost 3 years ago, when another protozoan came in. I only lost a couple fish that time, so it wasn't enough of a wake-up call for me.
This time was different. The 20g is now the QT, period.
One used to be able to reply on certain stores a bit, though that was still risky. But from my own experiences, and what I have read elsewhere, there seem to be more and more diseases getting into our fish these days. And at the same time, medications that used to work for this or that no longer do; bacteria as we all know can build up a resistance to antibiotics for example. Diagnosing a health issue is extremely difficult, and I rarely attempt it, which is why I so seldom enter such discussions; building up the knowledge base to be able to ascertain the issue and then know the best and safest treatment is a very scientific and long process. One of our forum members, Dawn, has spent more than 20 years closely studying this aspect of the hobby, and it was she who led me through this problem. She has not been as active on the general forum lately so I contacted her directly. She is an incredible wealth of knowledge in the area of fish nutrition and disease, and we are fortunate to have her with us.