After something did for 10 of our 12 guppies within a week, these two little guys are the only survivors. They must have been immune to whatever did for their tank-mates.
I guess it was either a parasite or a bacteria because all other parameters I can test for (nitrate, nitrites, ammonia etc) are fine. By the time I got hold of medicines it was too late.
Surprisingly sad.
Going to leave things to settle for a while then will re-stock and hope we'll do better.